feat: project search command with pre-processing hook

[DavidROliverBA]

Summary

Adds /arckit:search — keyword, type, and requirement ID search across all project artifacts. Fills the gap of having no search capability across 34+ document types.

• search-scan.mjs — UserPromptSubmit hook that indexes all ARC documents (metadata, content preview, requirement IDs)
• search.md — Command with ranked results, filter syntax, and handoffs
• Supports --type=ADR, --project=001, --id=BR-003 filters and combinations
• Scoring: title match (10pts), control fields (5pts), content (3pts), filename (2pts)

Files Changed

File	Change
arckit-claude/hooks/search-scan.mjs	New — UserPromptSubmit pre-processing hook
arckit-claude/commands/search.md	New — search command
arckit-claude/hooks/hooks.json	Register search hook
docs/guides/search.md	New — usage guide

Design Decisions

• No persistent index — scans fresh on each invocation for accuracy. Projects typically have <100 documents, so scan time is negligible.
• Hook does I/O, command does reasoning — follows the pattern established by health-scan.mjs and traceability-scan.mjs
• Content preview — extracts first 500 chars after document control table, giving Claude enough context to rank and excerpt results

Test plan

• JSON validation of hooks.json
• ESM import validation
• Search in a project with multiple document types
• Filter by type, project, and requirement ID
• Empty query shows usage help

🤖 Generated with Claude Code

[tractorjuice]

Superseded by #146, which includes your original search command, hook, and guide plus the missing deliverables:

• Converter output for Codex, OpenCode, and Gemini extensions
• Command count 57→58 across 44 files (READMEs, guides, HTML pages, plugin.json, extension manifests, CLAUDE.md)
• DEPENDENCY-MATRIX.md, CHANGELOG.md, docs/README.md updates
• Guide copied to plugin directory (arckit-claude/docs/guides/search.md)
• commands.html and index.html table rows

Your original commit is preserved as-is in #146. Thanks for the contribution!