chore(release): bump framework and 8 skills to v3.1.0 (MINOR)

- Define immutable MAM_VERSION="3.1.0" runtime constant in lib.sh
- Bump YAML frontmatter version to 3.1.0 across all 8 skill packages
- Document v3.1.0 changelog, 2-tier TUI readiness architecture, and migration guide in VERSIONS.md
- Document new tuning knobs (MAM_KILL_ON_READY_TIMEOUT, MAM_DIALOG_TAIL_LINES, MAM_READY_EMPTY_GIVEUP) in .mam.env.example
- Add 3-way lockstep and immutability test suite in tests/test_version_consistency.py

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

@@ -1,7 +1,7 @@
 ---
 name: multi-agent-mux-stop
 description: "Stop an agent herdr session (claude, antigravity/agy) and update .mam/agent-sessions.yaml. Default stops gracefully and marks status=stopped with conversation preserved for resume. Does NOT delete on-disk conversation artifacts (jsonl/db) — those are preserved unless --purge-conversation is passed. Use when ending a work session, switching to a different one, or cleaning up before a fresh start."
-version: 3.0.1
+version: 3.1.0
 author: godopu
 license: MIT
 platforms: [linux, macos]