docs: add T3 Code guide

[3metaJun]

mstack users who run Codex, Claude Code, Cursor, Grok, Pi or Antigravity inside T3 Code had no guidance. Each provider finds skills in different places, T3 rewrites $name differently per provider, and T3's own delegation and PR watching overlap with run-role and meta-mode's Bun watcher.

This adds docs/t3code.md (shipped under docs/) and a short "Use with T3 Code" section in the README. The guide covers where each provider looks for skills, install commands, composer invocation, gotchas (Claude's narrower roots, strict YAML in Cursor and Antigravity, Windows home differences, instance settings not inherited), worktree paths and what npm run audit-worktrees does with them, a t3.json setup script, orchestration overlap with guidance on what to prefer inside T3, and T3's safety rules. Antigravity and Grok harness names are not assumed; the Antigravity install uses an existing HARNESS_SKILLS_CODEX_DIR override (checked with --dry-run) so the page stays true whichever harness PR lands first.

Verification: claims were checked against the T3 Code source (commit 8f75697), grok inspect --json on Grok CLI 1.0.46, and tools/meta-mode/worktree-audit.mjs. Not verified, and stated as such in the page: whether interrupting a turn kills run-role children, how T3 reacts to a manually removed managed worktree, and Grok/Pi/OpenCode project roots. npm test passes (294 node tests plus validation).

Model: Claude Sonnet 5.5
Harness: Claude Code

🤖 Generated with Claude Code