Add Google Antigravity CLI as a new executor

[bborn]

Summary

• Adds antigravity as a new executor option alongside claude, codex, gemini, pi, opencode, openclaw — Google's terminal-first coding agent (agy) and the official successor to the Gemini CLI, which Google retires on 2026-06-18.
• Implements internal/executor/antigravity_executor.go modeled on the Gemini executor for tmux + process management, but adapted for what's actually documented today.
• Wires the executor into validation lists, completion, factory registration, and UI display name switches in cmd/task/, internal/ui/, and internal/db/.
• Documents the new executor in README.md, AGENTS.md, and docs/executor_interface.md.

Design notes

The Antigravity CLI is a TUI and does not currently expose documented command-line flags for headless prompting, session resume, or auto-approve ("YOLO") mode — auto-approve is configured in-app via /permissions. The executor reflects that reality:

• SupportsSessionResume() → false (retries replay the full prompt with appended feedback, same as Codex/Gemini fallbacks)
• SupportsDangerousMode() → false (no CLI flag exists; ResumeDangerous/ResumeSafe are no-ops with a log line)
• Task guidance is appended to the prompt (OpenClaw pattern) since agy has no separate system-prompt mechanism — and avoids clobbering an existing AGENTS.md in the worktree.

To stay resilient while Google stabilizes the CLI surface (this is agy 1.0.0), the invocation is overridable via env vars:

• ANTIGRAVITY_BIN — binary name/path (default agy)
• ANTIGRAVITY_PROMPT_FLAG — flag used to pass the initial prompt (default -i , matching the Gemini CLI it replaces)

IsAvailable() falls back to ~/.local/bin/agy since the official installer drops the binary there and that directory is not always on PATH for non-login shells.

Deprecating Gemini

Per the task, the gemini executor stays as-is until Google's Gemini CLI shutdown on 2026-06-18. The constant comment and README now note the planned deprecation.

Test plan

• `go build ./...`
• `go test ./internal/executor/...`
• `go test ./internal/db/...`
• `go test ./internal/ui/...`
• `go test ./cmd/task/...`
• `gofmt -l` (clean) and `go vet ./...` (clean)
• Manual smoke: install `agy`, create a task with `--executor antigravity`, verify the tmux pane launches `agy` with the prompt
• Confirm `ty create --executor antigravity` completion works (`completeFlagExecutors` now lists 7 entries)

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