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/fixtures/golden-install-parity/opencode.json

@@ -113,7 +113,7 @@
   "gsd-core/bin/gsd_run": "62d9b647ede212e6",
   "gsd-core/bin/shared/config-defaults.manifest.json": "517e6a7c1e9f4f16",
   "gsd-core/bin/shared/config-schema.manifest.json": "6bba2b9c9fa47cb8",
-  "gsd-core/bin/shared/model-catalog.json": "d0b59409a46a456a",
+  "gsd-core/bin/shared/model-catalog.json": "b55176ca044728d3",
   "gsd-core/bin/shared/runtime-aliases.manifest.json": "2df2c5ac1957911a",
   "gsd-core/bin/verify-reapply-patches.cjs": "caec5dbce11e3904",
   "gsd-core/contexts/dev.md": "dcb0de9dce33cf41",
@@ -360,7 +360,7 @@
   "gsd-core/workflows/scan.md": "ad8ebcad4626d4a8",
   "gsd-core/workflows/secure-phase.md": "e9a488cec3b4efdc",
   "gsd-core/workflows/session-report.md": "2e5b1205324ddefa",
-  "gsd-core/workflows/settings-advanced.md": "92524bd2a53b43f5",
+  "gsd-core/workflows/settings-advanced.md": "252b0d3edc315339",
   "gsd-core/workflows/settings-integrations.md": "d1711a95f44fdbde",
   "gsd-core/workflows/settings.md": "2e42ee34c791378a",
   "gsd-core/workflows/ship.md": "924c79e3cfd1e42b",