CB-632: rename the exported metrics bridged_* -> fleet_*

Part of #145 (CB-632). Doing this NOW, ahead of the rest of the path
renames, for one reason: the vms lead is about to wire a monitoring
dashboard to these series. Renaming a metric after a dashboard points at
it breaks continuity and silently leaves a dead panel. Renaming it before
costs nothing, so it goes first rather than at the cutover.

Nine series renamed, all declared in FleetMetrics.

Two real defects found while doing it:

  - FleetApp had "bridged_auth_failures_total" written as a LITERAL
    instead of using FleetMetrics.AUTH_FAILURES -- a second hand-written
    copy of a name, which is how these drift. It now uses the constant,
    so there is one source for that name again.
  - Nothing guarded the prefix. One test does assert a wire name
    (FleetAppAuthTest checks the real /metrics body for fleet_sessions),
    which is good, but it covers one series out of nine. The literal
    above was covered by nothing at all.

So this adds MetricNamesTest, which reads the constants reflectively
rather than listing them -- a test that lists the nine names is itself a
second hand-written copy, and would pass while a tenth went unchecked.
It asserts its own denominator too: "no name starts with bridged_" is
true of an empty set, so a sweep that found nothing would pass loudly.
Asserting the count of 9 makes a broken sweep fail instead.

Also renamed three herdr contract-test workspace labels, __bridged_* ->
__fleet_*. Those are throwaway workspaces created by ensureWorkspace, not
metrics, but they are the same word.

Verified: mvn clean install green, 52 classes, 881 tests, 0 failures.
Test count is up by 3 -- the new prefix, denominator and uniqueness
checks. No bridged_ string remains anywhere outside wiki/.

docs/CB-402-OpenCode-Adapter.md

@@ -301,5 +301,5 @@ identity (loopback peer PID → herdr pane) classified an **opencode** process a
 opencode-specific handling — confirming the identity model is peer-kind-agnostic, which is exactly
 what CB-308 needs when it stretches the roster across hosts.
 
-CB-502 counters for the same run: `bridged_sends_total{outcome="replied"} 1`,
-`bridged_replies_total{path="rendezvous"} 1`, `bridged_inbox_depth{...} 0`.
+CB-502 counters for the same run: `fleet_sends_total{outcome="replied"} 1`,
+`fleet_replies_total{path="rendezvous"} 1`, `fleet_inbox_depth{...} 0`.