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.

e2e/bridge_ask_transcript.md

@@ -0,0 +1,12 @@
+# Live bridge_ask — reverse rendezvous — 2026-07-16 16:30
+
+One worker paused its delegated turn to ask the primary, then resumed with the answer (profile `default`). Result: **`OK`**.
+
+## Round-trip
+
+1. **primary → worker** (delegation): the ask-forcing task.
+2. **worker → primary** (`bridge_ask`, 6.6s): 'PICK A COLOR: red or blue?' — surfaced on the primary's blocked send as a `question` with `turnId=term_656bb47d2c42a9e#1`.
+3. **primary → worker** (answer on that turn): `blue`.
+4. **worker → primary** (`bridge_reply`, 7.9s, source=reply): 'CHOSEN=BLUE'
+
+> **OK:** asked, resumed the same turn, and the reply reflected the primary's answer