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/mcp-server.cts

@@ -5,8 +5,16 @@
  * so any MCP-consuming host (Claude/Codex/OpenCode/VS Code/Gemini/Cursor/Cline/
  * Hermes) can drive GSD with NO bespoke plugin:
  *
- *   - point 1 (command): tool `gsd_invoke_command` → the command-routing hub
- *     (`createHub`/`dispatch`, src/command-routing-hub.cts).
+ *   - point 1 (command): tool `gsd_invoke_command` → `dispatchGsdCommand`
+ *     (src/shell-command-projection.cts), a bounded subprocess-shim to
+ *     gsd-tools.cjs. #2102 Stage 2: `commandRoutingHub.createHub()` called
+ *     with no args here always hit `if(!_cjsRegistry) return
+ *     makeUnknownCommand()` — every dispatch was UnknownCommand. No
+ *     fully-populated hub factory exists anywhere in gsd-core (every
+ *     createHub() caller builds a single-family hub for its own narrow
+ *     purpose), so the fix routes through the SAME shared dispatch helper
+ *     the pi extension uses (pi/gsd.cjs), mirroring the SUBPROCESS-REUSE
+ *     precedent already established for the OpenCode/Kilo hook bridge.
  *   - point 5 (state IO): tools `gsd_read_state` / `gsd_write_state` → the
  *     Phase 3 `stateIO` seam (src/state-io.cts, filesystem default).
  *
@@ -20,10 +28,11 @@
  */
 'use strict';
 
-// eslint-disable-next-line @typescript-eslint/no-require-imports
-import commandRoutingHub = require('./command-routing-hub.cjs');
 // eslint-disable-next-line @typescript-eslint/no-require-imports
 import stateIo = require('./state-io.cjs');
+// eslint-disable-next-line @typescript-eslint/no-require-imports
+import shellCommandProjection = require('./shell-command-projection.cjs');
+const { dispatchGsdCommand } = shellCommandProjection;
 
 export const PROTOCOL_VERSION = '2024-11-05';
 export const SERVER_NAME = 'gsd-core';
@@ -108,9 +117,11 @@ function callTool(name: string, args: unknown, ctx: McpContext): { content: Arra
       if (!family || !subcommand) {
         return { isError: true, content: [{ type: 'text', text: 'gsd_invoke_command requires string "family" and "subcommand".' }] };
       }
-      const hub = commandRoutingHub.createHub();
-      const res = hub.dispatch({ family, subcommand, args: Array.isArray(a.args) ? a.args : [], cwd, raw: undefined });
-      return { content: [{ type: 'text', text: JSON.stringify(res) }] };
+      const res = dispatchGsdCommand({ family, subcommand, args: Array.isArray(a.args) ? (a.args as string[]) : [], cwd });
+      if (!res.ok) {
+        return { isError: true, content: [{ type: 'text', text: res.stderr || res.stdout || `dispatch failed (exit ${res.code})` }] };
+      }
+      return { content: [{ type: 'text', text: res.stdout }] };
     }
     if (name === 'gsd_read_state') {
       const p = asString(a.path);