feat(profiles): add mcp pivot support via profile mcp key

[soulcodex]

Summary

Closes #39

• schemas/profile.schema.json — adds optional top-level mcp property (strategy: merge/replace + servers map) so profiles can declare MCP servers declaratively
• tooling/lib/mcp.sh — new --action seed (non-interactive) that reads mcp.servers from the profile and fans out to all present vendor config files with per-vendor translation:
  • .mcp.json (Claude): written verbatim
  • opencode.json: type stdio→local/http→remote, env→environment, command+args→combined array
  • .gemini/settings.json: type field removed, url→httpUrl for http entries
• tooling/lib/compose.sh — guarded seed call at end of compose (pure no-op when mcp key is absent — zero side effects on existing workflows)
• tooling/lib/test.sh — 7 new assertions T67–T73 covering .mcp.json creation, Opencode/Gemini translations, merge/replace strategies, and the no-op guard
• docs/mcp.md — new dedicated MCP reference page (vendor support matrix, per-vendor translation table, merge vs replace, env var convention, interactive wizard cross-link)
• docs/commands.md, docs/customization.md, docs/profiles.md, README.md — cross-links to docs/mcp.md

Test plan

• just lint — passes ✔
• just test — 229/229 pass ✔
• just index — rebuilt ✔

[soulcodex]

Closing in favour of a properly named branch.