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>

src/runtime-name-policy.cts

@@ -210,6 +210,7 @@ const RUNTIME_LABELS: Readonly<Record<string, string>> = {
   codebuddy: 'CodeBuddy',
   cline: 'Cline',
   zcode: 'ZCode',
+  pi: 'pi',
 };
 
 /**
@@ -258,6 +259,12 @@ const GLOBAL_CONFIG_HOME_FRAGMENTS: Readonly<Record<string, string>> = {
   cline:     "'.cline'",
   kimi:      "'.config', 'agents'",
   zcode:     "'.zcode'",
+  // pi's global config home is ~/.pi/agent (configHome: dot-home-nested,
+  // parent '.pi', name 'agent' — capabilities/pi/capability.json), matching
+  // resolveConfigHomeFromDescriptor's `path.join(home, parent, name)` for the
+  // no-probe dot-home-nested case (src/runtime-homes.cts). Two-segment
+  // path.join args, same shape as opencode/kilo/kimi above.
+  pi:        "'.pi', 'agent'",
 };
 
 /**
@@ -286,7 +293,7 @@ export function getGlobalConfigHomeFragment(runtime: string): string {
 // folds the shared-hooks-install skip).
 const RUNTIME_FLAG_IDS = Object.freeze([
   'opencode', 'kilo', 'codex', 'copilot', 'antigravity', 'cursor',
-  'windsurf', 'augment', 'trae', 'qwen', 'hermes', 'codebuddy', 'cline', 'kimi', 'zcode',
+  'windsurf', 'augment', 'trae', 'qwen', 'hermes', 'codebuddy', 'cline', 'kimi', 'zcode', 'pi',
 ] as const);
 
 /**