chore: untrack dev-only artifacts from git history

Remove from tracking (files remain on disk, now gitignored):
- .claude/ — Claude Code local settings and skills
- .specify/ — speckit planning tool
- specs/ — feature planning artifacts per iteration
- scripts/ — dev-only scripts (brainstorm, forge, sentinel)
- sentinel — dev script
- .idx/ — Firebase IDX local IDE config
- AGENTS.md, GEMINI.md, DESIGN.md — AI agent config files
- CONTRIBUTING.md — moved to local-only (Obsidian vault)
- docs/.obsidian/, docs/decisions/, docs/reports/, docs/operations/,
  docs/security/, docs/observability/, docs/pdr/, docs/process/,
  docs/specs/, docs/archive/, docs/assets/ — Obsidian vault content
- docs/doc-*.md, docs/dev-errors.md — personal dev notes

Repo now contains only: src/, tests/, prisma/, .github/, supabase/,
config files, CLAUDE.md, CHANGELOG, README, LICENSE.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: EmiyaKiritsugu3