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/lib.sh

@@ -25,6 +25,12 @@ SKILL_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 export PYTHONPATH="$SKILL_DIR:${PYTHONPATH:-}"
 WORKSPACE_ROOT="${WORKSPACE_ROOT:-$(cd "$SKILL_DIR/../.." && pwd)}"
 export WORKSPACE_ROOT
+
+# Framework semantic version. Single runtime source of truth; kept in lockstep
+# with VERSIONS.md and the 8 SKILL.md frontmatters by tests/test_version_consistency.py.
+# NOTE: unlike other MAM_* variables this one is intentionally NOT env-overridable.
+MAM_VERSION="3.1.0"
+export MAM_VERSION
 AGENT_SESSIONS_YAML="${AGENT_SESSIONS_YAML:-$WORKSPACE_ROOT/.mam/agent-sessions.yaml}"
 
 _sanitize_herdr_agent_name() {