fix: align grep symbol fields with backend

[jlitola]

Summary

• align code grep --symbol-field and MCP code_grep.symbol_fields with the deployed PkgSeer grepRepo contract
• centralize the exact 12 supported fields across validation, help, types, and GraphQL projection
• remove the underivable code, caller_count, and parent_symbol_ref fields
• update contract tests, implementation docs, and patch release fragments for githits and @githits/mcp

Verification

• bun test — 3,815 pass, 0 fail
• focused grep contract suite — 137 pass, 0 fail
• bun run typecheck
• bun run lint
• bun run format:check
• bun run build
• bun run validate:packages
• bun run plugins:generate && bun run plugins:check
• bun run smoke:cli — 93 steps passed
• bun run smoke:mcp — 50 steps passed
• targeted MCP descriptor agent eval — passed with zero failed calls or isolation violations
• authenticated live grep verified all 12 fields are accepted; content_hash and parent_path hydrate

Review

• internal Codex review: clean
• Claude Opus review: one low test-coverage finding fixed in 6758a98; follow-up clean