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>

tests/runtime-flags.test.cjs

@@ -14,7 +14,7 @@ const registry = require('../gsd-core/bin/lib/capability-registry.cjs');
 const EXPECTED_FLAGS = [
   'isOpencode', 'isKilo', 'isCodex', 'isCopilot', 'isAntigravity',
   'isCursor', 'isWindsurf', 'isAugment', 'isTrae', 'isQwen', 'isHermes',
-  'isCodebuddy', 'isCline', 'isKimi', 'isZcode',
+  'isCodebuddy', 'isCline', 'isKimi', 'isZcode', 'isPi',
 ];
 
 test('runtimeFlags: every known non-claude runtime sets exactly its own flag true', () => {
@@ -35,12 +35,12 @@ test('runtimeFlags: claude / unknown / empty → all flags false (fail-closed)',
   }
 });
 
-test('runtimeFlags: all 15 flags present + boolean + the object is frozen', () => {
+test('runtimeFlags: all 16 flags present + boolean + the object is frozen', () => {
   const flags = runtimeFlags('opencode');
   for (const f of EXPECTED_FLAGS) {
     assert.strictEqual(typeof flags[f], 'boolean', `${f} must be boolean`);
   }
-  assert.deepStrictEqual(Object.keys(flags).sort(), [...EXPECTED_FLAGS].sort(), 'exactly the 15 flags');
+  assert.deepStrictEqual(Object.keys(flags).sort(), [...EXPECTED_FLAGS].sort(), 'exactly the 16 flags');
   assert.ok(Object.isFrozen(flags), 'flags object must be frozen');
 });