feat(#2102): make pi a first-class installable runtime + fix its dispatch (ADR-1239)

Net-new EoS/pi installable runtime — purely additive (no prior runtime==='pi'
branches). pi is a bun-runtime programmatic-CLI whose /gsd command is registered
by a native ExtensionAPI extension and dispatches through the embedded engine.

Stage 1 (install plumbing):
- capabilities/pi/capability.json: full hostIntegration descriptor (imperative /
  slash-programmatic / active-model / native-extension / bun) + hostBehaviors
  {nativePlugin, pluginOnlyInstall}.
- --pi flag + interactive-menu renumber (All 17->18); pi added to RUNTIME_FLAG_IDS,
  RUNTIME_LABELS, RUNTIME_META, allRuntimes/runtimeMap, model-catalog defaults.
- Install mirrors OpenCode: pi installs the gsd.cjs extension + the shared engine
  payload (gsd-core + scripts + config markers) + the shared hooks bundle (spawned
  by the extension at lifecycle events, like OpenCode's plugin). pluginOnlyInstall
  EXCLUDES declarative command/agent/skill markdown, which pi has no host-read
  surface for (its /gsd is programmatic). _installNativePluginIfDeclared (extracted
  from the opencode-family path) copies pi/gsd.cjs -> ~/.pi/agent/extensions/gsd.cjs
  (global) / .pi/extensions/ (local). pi added to package.json files.
- Golden: new pi.json (320 files: extension + engine + 27-file hooks bundle, no
  markdown); the 16 other fixtures + claude-local change only by the shared
  model-catalog hash line.

Stage 2 (real dispatch + upgrades):
- Shared dispatchGsdCommand() (shell-command-projection): bounded, no-throw
  subprocess-shim to gsd-tools.cjs (the only full-surface dispatch path; no
  in-process full-hub factory exists). Fixes pi/gsd.cjs's createHub()-no-args bug
  (every dispatch was UnknownCommand) AND the identical bug in mcp-server.cts's
  gsd_invoke_command, which a vacuous unknown-family-only test had masked (now has
  a real dispatch regression test).
- pi/gsd.cjs: /gsd handler now (args, ctx) - tokenizes (quote-aware, via the
  shipped hooks/lib/git-cmd.js) + dispatches real family/subcommand (not hardcoded
  query/help); gsd_invoke gets a TypeBox (JSON-schema-fallback) parameters schema +
  consumes params; getArgumentCompletions; before_provider_request active-model
  steering (fail-open on null resolution); functional session_start /
  before_agent_start / session_before_compact hook bridges (spawn the shipped GSD
  hook scripts).
- EXTENSION_EVENT_SURFACES.pi expanded from ['tool_call'] to the full 30-event
  vocabulary.

Docs (host-integration matrix + how-to) + changeset (Added).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/how-to/install-on-your-runtime.md

@@ -461,6 +461,20 @@ GSD's hook-automation and native-MCP-registration integrations are not yet wired
 
 ---
 
+### pi
+
+```bash
+npx @opengsd/gsd-core@latest --pi --global
+```
+
+[pi](https://pi.dev) is a bun-runtime programmatic CLI whose extensions implement pi's own `ExtensionAPI` (`registerCommand`/`registerTool`/`registerProvider`/`pi.on`) rather than a settings-file or slash-markdown surface. GSD ships a single native-extension file:
+
+- **Extension** → `~/.pi/agent/extensions/gsd.cjs` (global) or `.pi/extensions/gsd.cjs` (local)
+
+The extension registers a `/gsd` command and a `gsd_invoke` tool that dispatch GSD commands via a bounded subprocess call to `gsd-core/bin/gsd-tools.cjs` (no fully-populated in-process command-routing hub exists — see the matrix's Stage 2 note). This is a **plugin-only install**: pi has no shared-settings hook surface (`hooksSurface: none`) and, unlike Claude/OpenCode/Kilo, no host-read markdown surface at all — pi's `/gsd` command is registered programmatically by the extension, not discovered from files, so GSD installs the extension plus its universal `gsd-core/` engine payload and the shared `hooks/`/`hooks/lib/` bundle (spawned by the extension itself, not by any config-file hook bus), and does **not** write any `commands/`, `agents/`, or `skills/` directory for pi. The extension bridges GSD's `session_start`/`before_agent_start`/`session_before_compact`/`tool_call` lifecycle events to those staged `hooks/` scripts as bounded, fail-open subprocesses, and steers pi's active model (`modelMode: active`) to a tier-resolved bare anthropic id via `pi.on('before_provider_request', ...)`. See the [`## pi`](host-integration-capability-matrix.md#pi) section of the host-integration capability matrix for the negotiated axes and citations.
+
+---
+
 ## Local vs global install
 
 All examples above use `--global`, which installs GSD once for your user account. To scope an install to a single project, replace `--global` with `--local`: