Untrack agent-dir .mcp.json — literal LLMMSG_HUB_BEARER inlined in 9 git-tracked files

All 9 agents/*/.mcp.json are git-tracked with the literal LLMMSG_HUB_BEARER inlined; none gitignored. Violates no-inlined-secrets-in-tracked-files. Fleet-wide hole (Pluto PLUTO-490, found during solo-config read; venus/ayudarg likely same). Fix = gitignore agents/*/.mcp.json + git rm --cached the 9 files (local copies persist, MCP unaffected). NO rotation (private repo + no-rotation rule, recover-and-reuse existing bearer). Env-var-reference/launcher-sourcing path crosses pm-llmmsgsrv-cc's maintenance boundary — OUT OF SCOPE; this WI is untrack-only. Reversible one-commit.