Workplan terminology: templates, updater guard, add_progress_event alias

- project_rules templates: rename workstream->workplan in prose; registration
  guidance is now file-first + fix-consistency C-06 (manual create_workplan/
  create_workstream calls create duplicates); progress examples use
  workplan_id; legacy field names (state_hub_workstream_id) annotated
- update_agent_instruction_files: never overwrite filled-in
  stack-and-commands/repo-boundary/architecture rules (TODO-marker guard)
- mcp_server: add_progress_event accepts workplan_id (preferred) with
  workstream_id kept as legacy alias, mirroring create_task

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

scripts/project_rules/agents-codex.template

@@ -33,8 +33,8 @@ statehub outbox status/replay after connectivity returns.
 # Offline brief — works without hub connection
 cat .custodian-brief.md
 
-# Active workstreams for this domain
-curl -s "http://127.0.0.1:8000/workstreams/?topic_id={TOPIC_ID}&status=active" \
+# Active workplans for this domain
+curl -s "http://127.0.0.1:8000/workplans/?topic_id={TOPIC_ID}&status=active" \
   | python3 -m json.tool
 
 # Check inbox
@@ -57,12 +57,12 @@ curl -s -X POST http://127.0.0.1:8000/progress/ \
     "summary": "what was done",
     "event_type": "note",
     "author": "codex",
-    "workstream_id": "<uuid>",
+    "workplan_id": "<uuid>",
     "task_id": "<uuid>"
   }'
 ```
 
-Omit `workstream_id` / `task_id` when not applicable.
+Omit `workplan_id` / `task_id` when not applicable.
 
 ### Update task status
 
@@ -86,7 +86,7 @@ curl -s -X PATCH "http://127.0.0.1:8000/tasks/<task_id>" \
 ## Session Protocol
 
 **Start:**
-1. `cat .custodian-brief.md` — domain goal and open workstreams (offline-safe)
+1. `cat .custodian-brief.md` — domain goal and open workplans (offline-safe)
 2. Check inbox: `GET /messages/?to_agent={REPO_SLUG}&unread_only=true`; mark read
 3. Scan workplans: `ls workplans/` — note `status: ready`, `active`, or `blocked` files and open tasks
 4. Check human-needed tasks: `GET /tasks/?needs_human=true`
@@ -145,7 +145,7 @@ owner: codex
 topic_slug: ...
 created: "YYYY-MM-DD"
 updated: "YYYY-MM-DD"
-state_hub_workstream_id: "<uuid>"   # written by fix-consistency — do not edit
+state_hub_workstream_id: "<uuid>"   # written by fix-consistency — do not edit (legacy name; holds the workplan id)
 ---
 ```