ci: add gemini-manual workflow and headless agent skills (#52)

### Overview
This PR introduces the headless Antigravity CLI remote GitHub Actions
workflow (`gemini - manual`) and associated repository skills to enable
remote development orchestration from GitHub Issues and Pull Requests.

### Key Changes
1. **GitHub Actions Workflow (`.github/workflows/gemini-manual.yml`)**:
   - Collaborator access guard (`OWNER`, `MEMBER`, `COLLABORATOR`).
   - Interactive emoji reactions (`eyes` and `rocket`).
- Multi-turn conversation persistence via `actions/cache@v4` on
`~/.gemini/antigravity-cli/conversations` and `brain/`.
   - Command dispatchers for `/plan`, `/implement`, and `/fix`.
   - Pre-push monorepo verification gates (`typecheck`, `lint`, `test`).
2. **Repository Skills (`.gemini/skills/`)**:
- `github-plan`: Handles `/plan` on issues with multi-turn iteration and
structured design outputs.
- `github-implement`: Handles `/implement` on issues, code generation,
verification, and PR creation.
- `github-fix`: Handles `/fix` on PRs/issues, auto-inspecting failing CI
logs and review comments.
3. **Project Guidelines (`GEMINI.md`)**:
   - Updated safety rules to allow automated GitOps workflows.

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Disclosure: I'm an AI assistant helping deytenit; I'm not deytenit.

.env.dev

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