#272: fleet_poll{target} is a drain, so gate it as one

fleet_poll is two operations behind one tool name. With `ticket` it observes
an async delegation and changes nothing. With `target` it calls
MessageService.drainReplies, which REMOVES the replies — a second call
returns nothing.

The handler gated both branches with a constant Authz.Action.READ, and did
not pass the target at all. READ is open to every authenticated role, so any
worker could read a peer's sessionId out of fleet_list and destroy the
replies that peer had queued for the primary. The gate failed open, and a
drained reply is not recoverable.

Three things already said the tight gate was intended:

  - fleet_ack, four lines below, gates the same drain as DRAIN, with a
    comment giving the exact reasoning missed here ("Acking removes a reply
    from the inbox, so it is a drain, not a read").
  - the REST path checks DRAIN in FleetApp.drainReplies.
  - wiki/2-Message-Server.md lists fleet_poll as lead-only, and the tool
    schema says "drain that worker's inbox".

Nothing that works today breaks: the documented flow is fleet_poll{target}
then fleet_ack{target,msgId}, and fleet_ack is already primary-only. A
worker could never complete that flow — only destroy its first half.

The required action is a function of the arguments, but the handler chose it
before looking at them. pollAction(target) makes that choice explicit. The
ticket branch stays READ on purpose: an architect may fleet_send, so it owns
tickets and must be able to poll them.

Why the suite missed it: FleetMcpAuthzTest checks every Action against every
Role, including "a worker may not DRAIN", and passed the whole time. The
policy table was right; the action fed to it was wrong, and nothing tested
that mapping. The new tests assert against pollAction itself, so the handler
keeps no private copy of the rule.

Mutation-proved: reverting pollAction to a constant READ turns exactly the
two new defect tests red and leaves the ticket-branch test green.

Introduced in 9daf1ec, where Authz.READ's own javadoc ("...task polling")
describes only the ticket half.

fleetd/src/main/java/dev/ltms/fleet/mcp/FleetMcp.java

@@ -319,10 +319,12 @@ public final class FleetMcp {
                 };
         BiFunction<McpSyncServerExchange, McpSchema.CallToolRequest, McpSchema.CallToolResult> pollHandler =
                 (exchange, req) -> {
-                    McpSchema.CallToolResult denied = deny(exchange, Authz.Action.READ, null);
-                    if (denied != null) return denied;
                     Map<String, Object> a = req.arguments();
-                    return poll(messages, str(a, "ticket"), str(a, "target"));
+                    String target = str(a, "target");
+                    // The action depends on the ARGUMENTS, not on the tool name -- see pollAction.
+                    McpSchema.CallToolResult denied = deny(exchange, pollAction(target), target);
+                    if (denied != null) return denied;
+                    return poll(messages, str(a, "ticket"), target);
                 };
         // CB-307 Increment 3: per-msgId ack (not needed in v1 but supported by the inbox).
         // Acking removes a reply from the inbox, so it is a drain, not a read.
@@ -724,6 +726,34 @@ public final class FleetMcp {
         return text("delivered to peer lead " + coordId + " (msgId " + msg.msgId() + ")");
     }
 
+    /**
+     * Which authorization action a {@code fleet_poll} call needs, decided by its arguments
+     * (fleetd #272).
+     *
+     * <p>{@code fleet_poll} is <strong>two operations behind one tool name</strong>. With {@code
+     * ticket} it observes an async delegation and changes nothing, which is a {@link
+     * Authz.Action#READ}. With {@code target} it calls {@link MessageService#drainReplies} on that
+     * session -- the replies are removed from the inbox and a second call returns nothing -- so it
+     * is a {@link Authz.Action#DRAIN}, the same gate {@code fleet_ack} already uses for removing a
+     * single message, and the same one the REST path uses at {@code FleetApp.drainReplies}.
+     *
+     * <p>Until this method existed the handler passed a constant {@code READ} for both branches.
+     * {@code READ} is open to every authenticated role, so any worker could read a peer's id out of
+     * {@code fleet_list} and destroy the replies that peer had queued for the primary. The gate
+     * failed open, and it did so because the required action is a function of the arguments while
+     * the handler chose it before looking at them.
+     *
+     * <p>The choice lives in this method, and not inline in the handler, so that a test can assert
+     * the mapping the handler actually uses. {@code FleetMcpAuthzTest} already checked every
+     * {@link Authz.Action} against every {@link Role} and passed throughout -- it tested the policy
+     * table, which was correct, while the defect was in which action the caller handed it.
+     *
+     * @param target the {@code target} argument of the call, or {@code null}/blank when absent
+     */
+    static Authz.Action pollAction(String target) {
+        return isBlank(target) ? Authz.Action.READ : Authz.Action.DRAIN;
+    }
+
     /** {@code fleet_poll}: check an async delegation by ticket, or drain a worker's inbox by target. */
     static McpSchema.CallToolResult poll(MessageService messages, String ticket, String target) {
         if (!isBlank(target)) {

fleetd/src/test/java/dev/ltms/fleet/mcp/FleetMcpAuthzTest.java

@@ -179,6 +179,53 @@ class FleetMcpAuthzTest {
                 "no CallerResolver supplied ⇒ authorization not enforced (legacy behaviour)");
     }
 
+    // --- which action each tool hands the gate (fleetd #272) ------------------------------------
+
+    /**
+     * fleetd #272: {@code fleet_poll{target}} drains a session's reply inbox, so it needs
+     * {@link Authz.Action#DRAIN} -- not the {@link Authz.Action#READ} the handler passed for both
+     * of its branches until this ticket.
+     *
+     * <p>This asserts against {@link FleetMcp#pollAction}, the method the handler itself calls, so
+     * the handler holds no separate copy of the rule that this test could miss. Every other test in
+     * this class checks the policy table (is a worker allowed to DRAIN?) and all of them passed for
+     * the whole time the defect was live -- the table was right, the action fed to it was wrong.
+     */
+    @Test
+    void pollingByTargetIsADrainAndPollingByTicketIsARead() {
+        assertEquals(Authz.Action.DRAIN, FleetMcp.pollAction("term_b"),
+                "poll by target removes the replies — that is a drain, not an observation");
+        assertEquals(Authz.Action.READ, FleetMcp.pollAction(null),
+                "poll by ticket changes nothing");
+        assertEquals(Authz.Action.READ, FleetMcp.pollAction("  "),
+                "a blank target is an absent target");
+    }
+
+    @Test
+    void aWorkerMayNotDrainAnotherSessionsInboxByPolling() {
+        FleetMcp m = mcp(true);
+
+        assertNotNull(m.denyFor(WORKER_A, FleetMcp.pollAction("term_b"), "term_b"),
+                "a worker draining a peer's inbox would destroy replies queued for the primary");
+        assertNotNull(m.denyFor(ARCH_DESIGN, FleetMcp.pollAction("term_b"), "term_b"),
+                "an architect has no lifecycle rights either — same gate as fleet_ack");
+        assertNull(m.denyFor(PRIMARY, FleetMcp.pollAction("term_b"), "term_b"),
+                "collecting a held reply is the primary's job");
+    }
+
+    /**
+     * The tightening must not close the branch that legitimately serves non-primary callers: an
+     * architect may {@code fleet_send}, so it owns tickets and must be able to poll them.
+     */
+    @Test
+    void pollingAnOwnTicketStaysOpenToWorkersAndArchitects() {
+        FleetMcp m = mcp(true);
+
+        assertNull(m.denyFor(WORKER_A, FleetMcp.pollAction(null), null));
+        assertNull(m.denyFor(ARCH_DESIGN, FleetMcp.pollAction(null), null),
+                "an architect delegates with wait:false, so it must be able to poll its ticket");
+    }
+
     // --- identity reconstruction from the transport context ------------------------------------
 
     @Test