Support `agy` (Antigravity CLI) as an architect

[mohidmakhdoomi]

Follow-up from #929 / #1059

Split out from #929 (which ships codex-only architect support). The Gemini CLI is retired 2026-06-18 (#778); its successor is Google's Antigravity CLI (agy), already used by Codev's consult lane. This issue tracks bringing agy to parity with claude/codex as a Codev architect.

Why it was deferred (the hard part)

agy has no system-prompt / model-instructions channel for injecting the architect role out-of-band, unlike the other harnesses:

• claude --append-system-prompt, codex -c model_instructions_file=, gemini (retired) GEMINI_SYSTEM_MD.
• agy's only per-session channel is --prompt-interactive "<role>", which puts the architect role in the first user turn — visible (agy "answers" it) and weaker / less durable than a system prompt over a long session.
• Other channels are unsuitable: the workspace AGENTS.md clobbers the committed file; the global ~/.gemini customizations root pollutes every agy session and races across architects.

Shipping a degraded architect was judged worse than none, so 929 went codex-only and this was split out. Validating role-holding quality is the crux of this issue.

What's already in place (reuse, don't rebuild)

• Harness seam (HarnessProvider, agent-farm/utils/harness.ts) including the engine-neutral buildResume crash-loop fix from Support codex as an architect (PIR #929) #1059 — a new AGY_HARNESS plugs in here.
• agy binary resolution already solved for consult: resolveAgyBin() / isRealAgyCli() / agyRespondsToVersion() in packages/codev/src/commands/consult/index.ts (guards against launching the Antigravity IDE launcher symlink instead of the headless CLI). Extract to a shared module and reuse.
• agy reads AGENTS.md natively (codex-like) — so no context/settings file is needed for project context; only the architect role needs a channel.

Scope (MVP)

1. Add AGY_HARNESS (keyed agy) selectable via .codev/config.json (shell.architect: "agy …" / shell.architectHarness: "agy").
   • Interactive launch: agy --dangerously-skip-permissions --prompt-interactive "<role>".
   • Role injection via the first-turn prompt (no system-prompt flag). Empirically validate role-holding — if poor, investigate framing the role as config, or wait for agy to add a system-prompt mechanism.
   • Binary resolution via the extracted resolveAgyBin guard.
   • Resume deferred: buildResume → null (fresh launch). agy supports --continue / --conversation for a later resume-parity pass.
   • resolveBinary? seam on HarnessProvider, applied at all architect executable-determination sites (no-Tower architect.ts, main + sibling launch in tower-instances.ts, reconnect / auto-restart in tower-terminals.ts).
2. doctor affirms agy as a supported architect.
3. Tests: agy arg construction, resume-skip → fresh launch, resolveBinary substitution incl. the reconnect path, harness detection.
4. Docs: arch.md architect-harness subsection.

Acceptance criteria

• afx architect (no-Tower), afx workspace start (clean + stale-Claude-.jsonl-present → no crash-loop, no --resume), and afx workspace add-architect all launch the real agy CLI (not the IDE) with the role applied.
• afx send single-line / multi-line (>3) / --interrupt / while-streaming deliver on agy's TUI (add a per-harness submit strategy if flaky).
• Tower stop→start reconnect + shellper auto-restart relaunch the agy architect (real bin).
• A builder spawned by an agy architect preserves CODEV_ARCHITECT_NAME affinity.

Recommended protocol

PIR — needs running-app validation of role-injection quality at the dev-approval gate (the reason this was split out).

References

• Support codex as an architect #929 — codex-only architect support (this is the deferred agy half)
• Support codex as an architect (PIR #929) #1059 — PR delivering Support codex as an architect #929 (harness seam + crash-loop fix to build on)
• Gemini CLI > Antigravity CLI - June 18, 2026 DEADLINE #778 — Gemini CLI → Antigravity CLI retirement (2026-06-18)
• af workspace failure with codex #591 — agent harness abstraction
• Multi-architect conversation resume: disambiguate via per-architect session UUID #832 — multi-architect resume (full resume-parity blocker)
• Harden harness resolution: unrecognized override command defaults to claude harness (residual --resume crash-loop) #1062 — unrecognized override commands default to claude harness