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