feat: add kit-kit meta-agent extension example

Port of Pi Pi (meta-agent with parallel expert subprocesses) to Kit's
extension system. Includes expert grid widget, query_experts tool,
custom footer, tool renderer, and orchestrator system prompt injection.

Also updates AGENTS.md with Yaegi gotchas, BubbleTea patterns, testing
recipes, and extension architecture notes.

Fixes golangci-lint issues: modernize min/max in overlay.go, replace
deprecated GetExtRunner() with new GetExtensionContext() SDK method,
remove broken --model flag from expert subprocess.

AGENTS.md

@@ -39,6 +39,53 @@ Keep this managed block so 'openspec update' can refresh the instructions.
 - Multi-provider LLM support via `llm.Provider` interface
 - MCP client-server for tool integration
 - Builtin servers: bash, fetch, todo, fs
+- **Extension system** (`internal/extensions/`): Yaegi-interpreted Go, 13 lifecycle events, custom tools/commands/widgets/overlays/editor interceptors
+- **TUI** (`internal/ui/`): Bubble Tea v2 parent-child model (`AppModel` → `InputComponent`, `StreamComponent`, etc.)
+- **Decoupling pattern**: `cmd/root.go` has converter functions (e.g. `widgetProviderForUI()`) that bridge `internal/extensions/` types to `internal/ui/` types — the UI never imports extensions directly
+
+## Key Patterns
+
+### Yaegi (Extension Interpreter) Gotchas
+- **No interfaces across boundary**: All extension-facing API types must be concrete structs, never interfaces. Yaegi crashes on interface wrapper generation.
+- **Function field bug**: Named function references assigned to struct fields return zero values across the interpreter boundary. Always use anonymous closure literals:
+  ```go
+  // WRONG: ctx.SetEditor(ext.EditorConfig{HandleKey: myHandler})
+  // RIGHT: ctx.SetEditor(ext.EditorConfig{HandleKey: func(k, t string) ext.EditorKeyAction { return myHandler(k, t) }})
+  ```
+- **Symbol exports**: Every new type exposed to extensions must be added to `internal/extensions/symbols.go`
+
+### BubbleTea Integration
+- **No `prog.Send()` from inside `Update()`**: Calling `prog.Send()` synchronously within a BubbleTea `Update()` handler deadlocks the event loop. Use `go appInstance.NotifyWidgetUpdate()` (async goroutine) instead.
+- **Height measurement**: `distributeHeight()` in `model.go` must measure using the same render path as `View()`. If an interceptor wraps rendering, measure with the wrapper too, or layout will mismatch.
+- **Channel-based prompts**: Extension prompt calls (PromptSelect, etc.) block on a `chan PromptResponse`. Extension slash commands run in dedicated goroutines (not `tea.Cmd`) to avoid stalling BubbleTea's Cmd scheduler.
+
+### Extension State Management
+- **Thread-safe maps on Runner**: Widget/header/footer/editor state lives on the Runner with `sync.RWMutex`, queried by UI via callbacks
+- **Context function fields**: The `Context` struct uses function fields (`Print func(string)`, `SetWidget func(WidgetConfig)`) wired by closures in `cmd/root.go`
+- **Package-level vars in extensions**: Yaegi supports package-level variables captured in closures — this is how extensions maintain state across event callbacks
+
+### Unicode in Widget Text
+- Widget content renders through `lipgloss.Style.Render()` which preserves ANSI escape codes
+- Use rune-based width calculations (`len([]rune(s))`) not byte length (`len(s)`) when aligning box-drawing characters or multi-byte symbols
+
+## Testing
+
+### Interactive TUI Testing with tmux
+Use tmux to test Kit interactively without blocking the agent:
+```bash
+tmux new-session -d -s kittest -x 120 -y 40 "output/kit -e examples/extensions/my-ext.go --no-session 2>kit_stderr.log"
+sleep 3
+tmux capture-pane -t kittest -p          # read screen
+tmux send-keys -t kittest '/command' Enter  # send input
+tmux kill-session -t kittest              # cleanup
+```
+
+### Non-Interactive Kit (Subprocess Spawning)
+Extensions can spawn Kit as a subprocess for sub-agent patterns:
+```bash
+kit --prompt "question" --quiet --no-session --no-extensions --system-prompt /path/to/prompt.txt --model provider/model
+```
+Key flags: `--quiet` (stdout only, no TUI), `--no-session` (ephemeral), `--no-extensions` (prevent recursive loading), `--system-prompt` (string or file path).
 
 ## External Repo Research
 - **ALWAYS use `btca`** to search external repos (e.g. iteratr, other reference codebases)