feat(gemini-cli): switch from extensions to subagents

Update Gemini CLI integration to support the new subagents feature:
- Update scripts/convert.sh to generate subagent .md files in integrations/gemini-cli/agents/
- Update scripts/install.sh to install subagents to ~/.gemini/agents/
- Update documentation in README.md and integrations/ to reflect the new structure
- Update .gitignore to exclude generated agent files
- Remove extension manifest generation as it's no longer needed for subagents

Closes #471

integrations/README.md

@@ -8,7 +8,7 @@ supported agentic coding tools.
 - **[Claude Code](#claude-code)** — `.md` agents, use the repo directly
 - **[GitHub Copilot](#github-copilot)** — `.md` agents, use the repo directly
 - **[Antigravity](#antigravity)** — `SKILL.md` per agent in `antigravity/`
-- **[Gemini CLI](#gemini-cli)** — extension + `SKILL.md` files in `gemini-cli/`
+- **[Gemini CLI](#gemini-cli)** — `.md` agent files in `gemini-cli/agents/`
 - **[OpenCode](#opencode)** — `.md` agent files in `opencode/`
 - **[OpenClaw](#openclaw)** — `SOUL.md` + `AGENTS.md` + `IDENTITY.md` workspaces
 - **[Cursor](#cursor)** — `.mdc` rule files in `cursor/`
@@ -104,9 +104,9 @@ See [antigravity/README.md](antigravity/README.md) for details.
 
 ## Gemini CLI
 
-Agents are packaged as a Gemini CLI extension with individual skill files.
-The extension is installed to `~/.gemini/extensions/agency-agents/`.
-Because the Gemini manifest and skill folders are generated artifacts, run
+Agents are packaged as Gemini CLI subagents.
+Subagents are installed to `~/.gemini/agents/`.
+Because the agent files are generated artifacts, run
 `./scripts/convert.sh --tool gemini-cli` before installing from a fresh clone.
 
 ```bash

integrations/gemini-cli/README.md

@@ -1,36 +1,40 @@
 # Gemini CLI Integration
 
-Packages all Agency agents as a Gemini CLI extension. The extension
-installs to `~/.gemini/extensions/agency-agents/`.
+Packages all Agency agents as Gemini CLI subagents. These agents
+install to `~/.gemini/agents/`.
 
 ## Install
 
 ```bash
-# Generate the Gemini CLI integration files first
+# Generate the Gemini CLI agent files first
 ./scripts/convert.sh --tool gemini-cli
 
-# Then install the extension
+# Then install them to ~/.gemini/agents/
 ./scripts/install.sh --tool gemini-cli
 ```
 
-## Activate a Skill
+## Use an Agent
 
-In Gemini CLI, reference an agent by name:
+In Gemini CLI, reference an agent by name in your prompt:
 
 ```
-Use the frontend-developer skill to help me build this UI.
+Use the frontend-developer agent to help me build this UI.
 ```
 
-## Extension Structure
+Or invoke the agent directly if your version of Gemini CLI supports it:
+
+```bash
+gemini --agent frontend-developer "How should I structure this React component?"
+```
+
+## Structure
 
 ```
-~/.gemini/extensions/agency-agents/
-  gemini-extension.json
-  skills/
-    frontend-developer/SKILL.md
-    backend-architect/SKILL.md
-    reality-checker/SKILL.md
-    ...
+~/.gemini/agents/
+  frontend-developer.md
+  backend-architect.md
+  reality-checker.md
+  ...
 ```
 
 ## Regenerate