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-create/scripts/create_session.sh

@@ -249,6 +249,7 @@ if [ "$TUI_READY_RC" -ne 0 ]; then
   {
     echo "=== MAM Diagnostic Dump ==="
     echo "Date: $(date -u +'%Y-%m-%dT%H:%M:%SZ')"
+    echo "MAM Version: ${MAM_VERSION:-unknown}"
     echo "Session: $SESSION_NAME"
     echo "Agent: $AGENT"
     echo "ReturnCode: $TUI_READY_RC"