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-status/SKILL.md

@@ -1,7 +1,7 @@
 ---
 name: multi-agent-mux-status
 description: "Read-only instant snapshot of all agent herdr sessions — name, YAML status, herdr alive, pane cmd/cwd, resume UUID on disk, and any drift. No mutation. Reuses reconcile.sh --dry-run for the diff logic. Use when you want to know 'what's running RIGHT NOW' without spinning up the monitor loop."
-version: 3.0.1
+version: 3.1.0
 author: godopu
 license: MIT
 platforms: [linux, macos]