fleetd #458: restate invariant 5 by purpose, not mechanism

Invariant 5 banned 'driving the terminal multiplexer directly', naming
herdr CLI and socket as the banned tool. That bans a mechanism. What it
protects is the control plane: nobody may move a fleet session, pane or
peer by a route that skips the bridge's policy checks.

In this repo herdr is itself the subject under test, so four contract
tests must open its socket on purpose (see the addendum's 'Herdr socket
tests' note). Under the old wording, a worker assigned to that code
reads invariant 5 and finds its only path to finish the task banned.

Restate the invariant by purpose: never move fleet state except through
the bridge. herdr's CLI and socket stay as the named example of the
banned route, not the definition of it.

CLAUDE.md

@@ -58,8 +58,9 @@ and the sender silently receives nothing. Fail toward the recoverable error.
    re-send because a call looks slow — the bridge delivers when the peer is `idle`, `blocked` or
    `done`. A spawned member must **also** have mounted the bridge MCP: until it has, it is not
    deliverable, and a send waits on that gate for ~60s and then fails without ever reaching its pane.
-5. **Never drive the terminal multiplexer directly** (no `herdr` CLI, no socket). The bridge owns
+5. **Never move a fleet session, pane or peer except through the bridge.** The bridge owns policy;
-   policy; the multiplexer owns PTYs. Going around the bridge bypasses every rule above.
+   the multiplexer owns PTYs. Any route that changes fleet state without the bridge's checks
+   bypasses every rule above — the `herdr` CLI and its socket are the usual example.
 
 ### Primary (lead) — run this on every task, in order