CB-622 lead follow-up: the three changes no worker could make

1. opencode.json — mount key bridged -> fleetd. Unit C could not do this:
   the file is neutralized by the worktree overlay, so every worker sees a
   stub and correctly reported it held no mount key. Tracked as CB-628.

2. e2e/bridge_ask_transcript.md keeps its name. It is a dated record of a
   run on 2026-07-16 that really did call bridge_ask, and its first line
   says so. The harness now writes fleet_ask_transcript.md for new runs;
   its header already says 'Live fleet_ask', so the two now agree.

3. docs/MCP-Contract.md line 20 — the historical banner describes section
   6, and section 6 now says fleet_ask.

docs/MCP-Contract.md

@@ -17,7 +17,7 @@
 > checked against `mcp/BridgeMcp.java` on 2026-08-17 and are accurate.
 >
 > What is still worth reading here is **§6 — the flows and the error model** (rendezvous,
-> `bridge_ask`, detached delivery, the turn-done fallback). The shapes it describes are the ones
+> `fleet_ask`, detached delivery, the turn-done fallback). The shapes it describes are the ones
 > that shipped; only the names around them drifted. Rewriting this page is tracked as **CB-609**.
 
 `bridged` is the **sole communication gateway** for every Claude session in the bridge. Both