pi: add agent config, themes, subagents, and extensions

.pi/agent/extensions/subagent/README.md

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+# Subagent roles for pi
+
+Installed roles:
+
+- `reviewer` — review changes, bugs, risks, maintainability
+- `librarian` — explore and map a codebase
+- `uiux-designer` — UI/UX design direction, flows, states, and implementation guidance
+
+## Commands
+
+- `/subagents` — list discovered user-level agents
+- `/subagents both` — include project-local `.pi/agents`
+
+## Example prompts
+
+- `Use the subagent tool with reviewer to inspect the auth refactor.`
+- `Use subagent in parallel: librarian maps the settings flow, uiux-designer proposes an improved UX direction, reviewer lists product and implementation risks.`
+- `Chain librarian -> uiux-designer to redesign onboarding. Use {previous} in the second task.`
+
+## Notes
+
+- User agents live in `~/.pi/agent/agents`
+- Project-local agents can live in `.pi/agents`
+- Project agents require `agentScope: "project"` or `"both"`
+- Parallel mode allows up to 8 tasks and runs up to 4 concurrently
+- `reviewer` uses `openai-codex/gpt-5.4:xhigh`
+- `librarian` uses `anthropic/claude-sonnet-4-6`
+- `uiux-designer` uses `google-antigravity/gemini-3.1-pro-high`