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-resume/SKILL.md

@@ -1,7 +1,7 @@
 ---
 name: multi-agent-mux-resume
 description: "Resume an existing agent (claude, antigravity/agy) conversation by UUID into a herdr session. Reads .mam/agent-sessions.yaml for the saved session/conversation id, spawns (or reuses) a herdr session of the matching name, and runs `claude -r <id>` or `agy --conversation <id>` inside. Use when you want to reattach to a previous session's context, or revive a session whose herdr died but the agent's conversation is still on disk."
-version: 2.0.0
+version: 2.1.0
 author: godopu
 license: MIT
 platforms: [linux, macos]