feat(install): add antigravity and grok harness targets

[3metaJun]

Problem

mstack could not install into Antigravity (agy) or Grok, two providers T3 Code runs. Antigravity reads user skills only from ~/.gemini/antigravity-cli/skills and ~/.gemini/config/skills, never ~/.agents/skills. Grok reads ~/.agents/skills, so it should reuse the shared copy.

Solution

• Add antigravity (own root, HARNESS_SKILLS_ANTIGRAVITY_DIR) and grok (shared copy, HARNESS_SKILLS_GROK_DIR) to profiles/harnesses.json, with empty adapters because the canonical skills already carry name. Cursor already reads ~/.agents/skills, so it gets a docs note only.
• Both CLIs take the prompt as the value of a flag (agy --print, grok --single), so --model placed before the prompt was swallowed as the prompt. The runtime schema gains optional promptArgs, which run-role places directly before the prompt.
• Read-only args: Grok uses --tools read_file,list_dir,grep --disallowed-tools search_tool,use_tool (CLI-enforced; a shell-write prompt had no shell tool, and the tool list showed only those three). Antigravity uses --mode plan, which is mode-level only: it left the workspace untouched but still wrote its own plan file under ~/.gemini/antigravity-cli/. README documents this limit next to OpenCode's.
• Update install usage text, models.example.json, environments.example.json, README and docs/harness-adapters.md (Grok, Antigravity and Cursor rows with sources).
• Left alone: profiles/upstreams.json and docs/guide/01-setup.md still say <codex|claude|opencode|pi>. They are hash-gated by upstream-manifest.json.
• Unverified: whether agy discovers agent files in ~/.gemini/antigravity-cli/agents. The README says so.

Verification

• npm test: 298 pass, 0 fail. npm run check-package passes.
• Tests set the new HARNESS_SKILLS_* env vars so they never write to the real home. New coverage: default and overridden roots, shared Grok copy vs separate Antigravity copy, artifact roots, and run-role argument order.
• Live: run-role --read-only --execute and smoke-harnesses --execute passed for agy 1.2.17 (--model gemini-3.8-flash-medium) and grok 1.0.46 (--model grok-4.5), including a file read under the read-only args.

Model: Claude Sonnet 5.5
Harness: Claude Code

🤖 Generated with Claude Code