feat(#2103): drive VS Code through the Embeddable Orchestration System (ADR-1239)

VS Code is a net-new EoS runtime that — unlike every prior migration — is NOT
CLI-installed (Marketplace/VSIX extension). It has zero runtime==='vscode'
branches in bin/install.js and stays that way (regression-guarded); it is driven
entirely through the negotiated imperative Host-Integration adapter.

Registry + validator (the hard part):
- capabilities/vscode/capability.json (role:runtime): full hostIntegration block
  (imperative / palette / active vscode.lm model / engine hook bus /
  sandboxed-storage / mcp transport / sandboxed-web runtime; dispatch nested,
  maxDepth 5 per VS Code's documented subagent depth).
- capability-validator.cjs extended so a role:runtime capability can legitimately
  declare "extension-distributed, no config directory": new configHome.kind:'none'
  + installSurface:'none' (+ GATE-A pairing + the parity maps), with localConfigDir
  and configHome.name made conditional on kind!=='none'. All 18 runtimes still
  validate; getDirName returns a distinct sentinel (not '.claude') for a no-config
  runtime.
- The add-a-registry-runtime tax: NON_INSTALLABLE_RUNTIMES exemption in the
  runtime-flags drift guard, vscode added to global-config-home SPECIAL_CASED,
  EXPECTED_PROFILES.vscode='ide', and the config-adapter/derivation/pin-count
  guards updated. No golden-install fixture, model-catalog, or CONFIGURATION rows
  (vscode never enters allRuntimes).

Dispatch + extension surface:
- Fixed vscode/extension.js's createHub()-no-args bug (every dispatch was
  UnknownCommand, masked by a vacuous reachability test) — now reuses the shared
  dispatchGsdCommand subprocess-shim (Node/desktop); the reachability test is
  tightened to assert real dispatch.
- Promoted the #1933 host binding to a shipped vscode/host-binding.js; activate()
  now composes the model/hookBus/stateIO seams through it. Corrected the model
  seam to VS Code's real API (vscode.lm.selectChatModels() -> model.sendRequest();
  vscode.lm.sendRequest does not exist) so the binding actually composes on real
  desktop VS Code instead of throwing.
- New vscode/browser.js Web Extension entry with ZERO Node APIs (the engine's
  config/capability loading is Node-bound, so the web entry registers the surface
  and directs full dispatch to the native MCP server — honestly documented).
- UPGRADE 1: GSD skills as native Language Model Tools (contributes.languageModelTools
  + vscode.lm.registerTool), invoke() dispatching through the hub.
- UPGRADE 2: native subagent dispatch wired onto #runSubagent /
  chat.subagents.allowInvocationsFromSubagents (fail-soft on API availability,
  maxDepth 5 enforced).
- vscode/package.json: browser entry, engines.vscode ^1.105, chatParticipants +
  languageModelTools contributions; fixed a stale activationPoints->activationEvents
  manifest key. Added "vscode" to the package files array.

Docs (## vscode matrix section) + changeset (Added).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/gemini-runtime-removed.test.cjs

@@ -164,15 +164,21 @@ describe('#1928 gemini removed from every runtime-name-policy surface', () => {
     assert.strictEqual(getRuntimeNewProjectCommand('gemini'), '/gsd-new-project', 'new-project override removed → default');
   });
 
-  test('runtimeFlags has no isGemini and covers exactly the non-claude registry runtimes (count-agnostic)', () => {
+  test('runtimeFlags has no isGemini and covers exactly the non-claude, CLI-installable registry runtimes (count-agnostic)', () => {
     const flags = runtimeFlags('claude');
     assert.ok(!('isGemini' in flags), 'isGemini flag must be gone');
     // The flag set tracks the non-claude registry runtimes (one is<Runtime> per
     // id), so adding a runtime updates the count automatically — no hand-pinned
     // number that would break on the next runtime addition.
-    const expectedNonClaudeCount = Object.keys(registry.runtimes).filter((id) => id !== 'claude').length;
+    // #2103: registry runtimes with installSurface === 'none' (e.g. vscode —
+    // Marketplace/VSIX-distributed, never CLI-installed) have no --<rt> flag
+    // by design (see tests/runtime-flags.test.cjs's NON_INSTALLABLE_RUNTIMES)
+    // and are excluded from this count too.
+    const expectedNonClaudeCount = Object.keys(registry.runtimes)
+      .filter((id) => id !== 'claude' && registry.runtimes[id].runtime.installSurface !== 'none')
+      .length;
     assert.strictEqual(Object.keys(flags).length, expectedNonClaudeCount,
-      'flag count must equal the non-claude registry runtime count');
+      'flag count must equal the non-claude, CLI-installable registry runtime count');
   });
 
   test('gemini no longer maps to GEMINI.md (defaults to AGENTS.md)', () => {