refactor: standardize --agent option usage, improve YAML registry fallback, and fix layout falsy-zero trap (J-1)

- In stop_session.sh and update_yaml_resumed.sh: standardize explicit --agent option and use resolve_agent_type_from_registry to read agent type from YAML/DB state rather than brittle suffix-only regex inference.
- Update multi-agent-mux-stop/SKILL.md, multi-agent-mux-resume/SKILL.md, multi-agent-mux-create/SKILL.md, and deploy/INSTALL.md to standardize passing --agent explicitly.
- Fix J-1 in layout.py: refactor _env_int(*names, default=None) to take an explicit default parameter, eliminating the falsy-zero trap so MAM_MIN_PANE_COLS=0 is respected.
- Add regression and contract tests: test_j1_env_zero_min_cols_matches_flag_zero, test_j1_env_zero_min_rows_matches_flag_zero, test_comp_stop_agent_fallback_*, test_comp_docs_stop_examples_pass_agent.
- Verified 100% UNANIMOUS PASS from Planner claude and Reviewers claude and cline.

.agents/skills/multi-agent-mux-stop/SKILL.md

@@ -37,6 +37,7 @@ The stop command is always **graceful by default**:
 
 ```bash
 SESSION_NAME=<workspace>-creator-<agent>  # convention
+AGENT=claude                              # claude | agy | hermes | cline — always pass it
 AGENT_SESSIONS_YAML=.mam/agent-sessions.yaml
 
 # 1) Session is registered?
@@ -66,20 +67,27 @@ fi
 ```bash
 # 1. Stop gracefully (default — captures ID, shuts down safely, status=stopped)
 bash .agents/skills/multi-agent-mux-stop/scripts/stop_session.sh \
-  --session "$SESSION_NAME"
+  --session "$SESSION_NAME" --agent "$AGENT"
 
 # 2. Stop gracefully + record a custom stop reason
 bash .agents/skills/multi-agent-mux-stop/scripts/stop_session.sh \
-  --session "$SESSION_NAME" --reason api_error
+  --session "$SESSION_NAME" --agent "$AGENT" --reason api_error
 
 # 3. Stop gracefully + clean up on-disk conversation (DANGEROUS)
 #    — this prevents any future resume (status=terminated, resumable=false).
 bash .agents/skills/multi-agent-mux-stop/scripts/stop_session.sh \
-  --session "$SESSION_NAME" --purge-conversation
+  --session "$SESSION_NAME" --agent "$AGENT" --purge-conversation
 ```
 
 **Idempotency**: if the row is already `status: stopped`, the script prints `already stopped (...)` and exits 0 — re-running is a safe no-op.
 
+**`--agent` is the standard.** Pass it on every invocation. If omitted, the script
+resolves the agent from the registry record — the row's `agent` field, then the
+session-name suffix, then `pane.cmd` — and exits 2 if none of the three resolve.
+The fallback exists for recovery, not as the normal calling convention: a session
+whose name carries no agent suffix (e.g. `agy-creator-01`) is only resolvable
+while its registry row survives.
+
 ### State machine
 
 ```