chore(release): bump framework and skills version to v2.1.0

- Bump all 8 SKILL.md frontmatter version fields to 2.1.0
- Add v2.1.0 changelog in VERSIONS.md highlighting 2xK grid layout engine (B-20), explicit --agent standardization, and --herdr-session option hardening
- Update skills version matrix in VERSIONS.md

.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: 2.0.0
+version: 2.1.0
 author: godopu
 license: MIT
 platforms: [linux, macos]