docs: GitNexus and Brain MCP reference slice

[litbitrim]

Scope

• Freeze the observed 13 GitNexus MCP tools against the merged PlugBrain 0.5 source list of 31 tools.
• Record concrete capability gaps and source references for M09/M18.

Evidence

• Commit c941af6.
• Exact tool names and counts checked against the captured GitNexus JSON and PlugBrain MCP_TOOLS source.
• git diff --cached --check passed before commit.

Open gates

• Independent provider review did not complete: Google review lanes returned upstream 503, rate limit, or no verdict.
• M18 parity and installed 0.5 behavior are unverified. Keep this PR draft until review and exact-source CI pass.