docs: add agent inbox check to session protocol

Check get_messages(to_agent="activity-core") at session start per CUST-WP-0015.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CLAUDE.md

@@ -46,6 +46,12 @@ get_domain_summary("custodian")   # workstreams, blocking decisions, recent prog
 ```
 If the call fails, the API is offline: `cd ~/the-custodian/state-hub && make api`
 
+**Step 1b — Check the agent inbox**
+```
+get_messages(to_agent="activity-core", unread_only=True)
+```
+Mark messages read with `mark_message_read(message_id)`. Act on any coordination requests before proceeding.
+
 **Step 2 — Scan local workplans**
 
 Read every `.md` file under `workplans/`. For each file with `status: active`, extract and note:
@@ -170,6 +176,24 @@ make ingest-sbom REPO=activity-core SCAN=1 REPO_PATH=$(pwd)
 
 ---
 
+### Ralph Loop — Workplan-Tied Usage
+
+**Rule: always use `/ralph-workplan` instead of `/ralph-loop` directly.**
+
+```
+/ralph-workplan workplans/<ID>-<slug>.md [--max-iterations 20]
+```
+
+This skill guards against runaway loops:
+1. **Refuses to start** if the workplan `status` is already `done`
+2. **Self-retires** — re-reads the workplan file each iteration; outputs `<promise>HEUREKA</promise>` the moment all tasks are `done`
+3. Always sets `--completion-promise HEUREKA` and a bounded iteration count
+
+**Never** start a ralph loop with a raw static implementation prompt. A static prompt
+has no completion awareness and will loop forever even after the work is done.
+
+---
+
 ### Quick Reference
 
 `~/the-custodian/state-hub/mcp_server/TOOLS.md` — compact MCP tool reference