Use Cases: invariant 5 states a purpose, not a banned tool

Propagates the canonical block change merged in the repo as eccd054
(fleetd #458). Invariant 5 was:

  Never drive the terminal multiplexer directly (no herdr CLI, no socket).

It banned a mechanism. What it is for is banning a control plane, and the
two readings pick out the same actions in every project except the bridge's
own repo, where the multiplexer is the subject under test. Four contract
tests open the herdr socket on purpose, so a worker assigned to herdr code
read invariant 5 and found the only action that could finish its task was
banned. It now reads:

  Never move a fleet session, pane or peer except through the bridge.

The herdr CLI and its socket stay in the text, as the usual example of the
banned route rather than as the ban itself.

The sync check in CLAUDE.md prints "in sync: True" against this file. Block
size 17557 characters / 17718 bytes.

7-Use-Cases.md

@@ -363,8 +363,9 @@ 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
-   policy; the multiplexer owns PTYs. Going around the bridge bypasses every rule above.
+5. **Never move a fleet session, pane or peer except through the bridge.** The bridge owns policy;
+   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