docs: make repository docs canonical

[joeblau]

Summary

• make /docs the canonical, GitHub-rendered documentation source
• replace GitBook directives and generated SUMMARY metadata with standard Markdown
• add root README navigation and repository-wide CODEOWNERS for @joeblau
• add Bun-driven index/format checks and a pinned external-link workflow
• exclude ignored agent worktrees from the root TypeScript project

The empty GitHub Wiki has been disabled so contributors have one source of truth.

Verification

• bun run check
• bun install --frozen-lockfile
• markdownlint: 0 issues across 13 files
• docs index: all 11 pages covered
• Lychee: 166 links checked, 0 broken
• GitBook directives: 0
• details sections: 17 opening / 17 closing