CB-622: rename the opencode mount key bridged -> fleetd

Unit C could not make this change. The tracked opencode.json is neutralized
by the worktree overlay (GitWorktrees.WORKTREE_HOSTILE_CONFIGS), so every
worker sees a stub instead of the real file and correctly reported that it
held no mount key. The lead has to make this one by hand, the same way
bridged.yaml is handled.

Takes effect for an opencode lead on its next session start.

opencode.json

@@ -4,7 +4,7 @@
     "CLAUDE.md"
   ],
   "mcp": {
-    "bridged": {
+    "fleetd": {
       "type": "remote",
       "url": "http://127.0.0.1:8765/mcp",
       "enabled": true