feat(install): add project-scope install

[3metaJun]

Problem

The installer only writes user-level skill directories. T3 Code runs each thread in a per-thread git worktree, and several providers read project skills only from the thread's cwd (Claude: .claude/skills; Cursor and Antigravity: .agents/skills and others). There was no way to install mstack into a repository.

Solution

--project <dir> copies skills into each harness's project root, defined as a project field in profiles/harnesses.json:

• .agents/skills for Codex, OpenCode, pi, Grok and Antigravity (one shared copy; paths from each harness's docs and T3's AntigravitySkills.ts)
• .claude/skills for Claude, with the unchanged directory-name adapter output

Staging, locking, receipts, rollback and --replace backups are unchanged. Backups land in <repo>/.agents/.harness-skills-backups/ and <repo>/.claude/.harness-skills-backups/, never under a skills root. Guardrails: the directory must exist, and the home directory, paths inside the mstack package, and .harness-skills-* paths are refused. --artifact, --migrate and --environment (including SSH) are refused with --project; HARNESS_SKILLS_*_DIR overrides are ignored. Empty .harness-skills-stage shells are now pruned after staging. README, docs/harness-adapters.md, docs/t3code.md and the usage text document the roots and the rule that a new worktree contains only committed files (T3 uses git worktree add from a ref).

Verification

• npm test (422 pass) and npm run check-package
• New tests use temp dirs only: per-harness roots, shared-group dedupe, Claude adapter output, refusal cases, rollback on a failed later target, --replace backup location, dry-run output, and no writes to the home directory.

Model: Claude Sonnet 5.5
Harness: Claude Code

🤖 Generated with Claude Code