Add code-map plugin

[annyeong844]

Adds a catalog entry for code-map — token-efficient, drift-resistant code reads for coding agents.

• Source: https://github.com/annyeong844/map (MIT), pinned to a full commit SHA.
• What it is: grep finds; code-map's read pulls the exact symbol slice by coordinate and re-anchors it when files move (0 silently-wrong bytes at churn scale). Bundles a retrieval-routing skill. TS/JS + Python, one runtime dependency.
• Components: one MCP read tool + a code-map-retrieval skill.
• Evidence: reproducible benchmarks at https://github.com/annyeong844/code-map-bench (verifier included).

Checklist:

• One entry in .grok-plugin/marketplace.json, valid JSON, kebab-case unique name.
• Remote source pins a full 40-char lowercase SHA (public, reachable).
• .grok-plugin/plugin-index.json regenerated (validate-catalog + index --check pass).
• homepage + clear description + brand-scoped keywords + category.
• Licensed (MIT).