Add the agy Harness (Antigravity CLI) #436

Parent

#432

What to build

harness agy runs a Command's sessions on Antigravity CLI (Gemini 3.8 Flash and the other Models agy offers), wherever claude and codex can be chosen: the harness flag, harness.default, [harness.agy] with a Model and Effort, and Setup. It is the first new Harness, so it also brings in two shared pieces:

• the instruction-file fallback link;
• Setup listing only installed Harnesses.

Facts: the harness candidates research note, §1 and §6. Flags and environment: ADR 0013.

Acceptance criteria

• Each session runs agy -p with --dangerously-skip-permissions and --output-format stream-json, never --sandbox, with stdin null and AGY_CLI_DISABLE_AUTO_UPDATE=true in its environment. The same applies to every Resume and to the check.
• The Factory skills are linked into .agents/skills/, and the prompt loads its skill with the /thirdshift-<skill> first line.
• When the worktree root has CLAUDE.md and has neither AGENTS.md nor GEMINI.md, GEMINI.md is linked to CLAUDE.md there and kept out of git. When either file exists, nothing is linked.
• The Model and Effort are checked against agy models and its supported efforts before any work, regardless of case. A bad Model or Effort fails, naming the valid ones.
• A stream parser gives the progress lines, and the session id from init. From the final result it gives the final message, the status, the error and the usage. An ERROR status, or a non-zero exit, fails the session with agy's error.
• A Resume runs --conversation <id>.
• A fake agy in the test harness emits agy's stream shape. A sessions scenario test covers the flags, the environment, skills, the fallback link, the check, Resume, a failed turn, and the final message reaching an Architecture review.
• Setup lists only the installed Harnesses, with claude as the default when it is installed. The User config template has a section for every Harness.
• The docs list agy: its product, CLI, flags, environment, instruction files (it doesn't read ~/.claude/), and its slow, variable start-up.

Blocked by

• Gather each Harness into one adapter #434