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/reference/capability-matrix.md

@@ -72,7 +72,7 @@ points.
 | `tdd` | feature | full | `>=1.6.0` | `plan:pre`, `execute:post` | contribution, gate | first-party |
 | `ui` | feature | full | `>=1.6.0` | `plan:pre`, `execute:wave:post`, `verify:post` | step, gate | first-party |
 
-### Runtime capabilities (role: runtime) — 16
+### Runtime capabilities (role: runtime) — 17
 
 Runtime capabilities adapt GSD to a specific AI runtime or IDE — emitting
 skills, agents, hooks configuration, and surface files for that host. They
@@ -93,6 +93,7 @@ emission), so their extension-point and hook-kind cells are `—`.
 | `kilo` | runtime | core | `>=1.6.0` | — | — | first-party |
 | `kimi` | runtime | core | `>=1.6.0` | — | — | first-party |
 | `opencode` | runtime | core | `>=1.6.0` | — | — | first-party |
+| `pi` | runtime | core | `>=1.7.0` | — | — | first-party |
 | `qwen` | runtime | core | `>=1.6.0` | — | — | first-party |
 | `trae` | runtime | core | `>=1.6.0` | — | — | first-party |
 | `windsurf` | runtime | core | `>=1.6.0` | — | — | first-party |