refactor(#1679): ADR-1239 Phase B — collapse program + command chains [AC2 slice 4] (#1813)

* refactor(#1679): ADR-1239 Phase B — collapse program + command chains [AC2 slice 4]

Phase 2 AC2 slice 4. Collapses two more duplicated runtime->string chains in
bin/install.js's post-install next-step message:

- program (14 branches): an EXACT duplicate of runtimeLabel -> getRuntimeLabel.
- command (14 branches): the per-runtime /gsd-new-project invocation syntax
  (gemini '/gsd:', codex '$', cursor skill-mention, kimi '/skill:', default
  '/gsd-new-project') -> new getRuntimeNewProjectCommand(runtime) helper.

- src/runtime-name-policy.cts: RUNTIME_NEW_PROJECT_COMMANDS table +
  getRuntimeNewProjectCommand(runtime) (sibling to runtimeFlags/getRuntimeLabel).
- bin/install.js: import getRuntimeNewProjectCommand; replace the program +
  command chains with single lookups.
- tests/runtime-label-policy.test.cjs: 2 new tests for
  getRuntimeNewProjectCommand (4 overrides + default for the other 12).

runtime === count: 53 -> 25 (-28). Cumulative Phase 2 this session: 129 -> 25
(-104). golden-install-parity 16/16 (program/command are stdout-only so not
parity-covered, but program matches RUNTIME_LABELS exactly and command values
are preserved verbatim in the table). AC2 data-collapse now essentially
exhausted; remaining 25 branches are the ADR-1235 agent-loop tail + per-runtime
semantic behavior.

* chore(changeset): add Changed fragment for program+command collapse (#1679)

tests/runtime-label-policy.test.cjs

@@ -33,7 +33,7 @@ const assert = require('node:assert/strict');
 const runtimeNamePolicy = require('../gsd-core/bin/lib/runtime-name-policy.cjs');
 const registry = require('../gsd-core/bin/lib/capability-registry.cjs');
 
-const { getRuntimeLabel } = runtimeNamePolicy;
+const { getRuntimeLabel, getRuntimeNewProjectCommand } = runtimeNamePolicy;
 
 // Golden oracle: hardcoded expected map of all 16 runtime ids to their short
 // install/uninstall display label. A pinned expected value in a TEST is correct
@@ -101,3 +101,30 @@ test('getRuntimeLabel fallback: alias is NOT auto-expanded (raw id match only)',
   assert.strictEqual(getRuntimeLabel('claude-code'), 'Claude Code',
     'getRuntimeLabel("claude-code") must return the default "Claude Code" (raw-id match only; aliases are not expanded)');
 });
+
+// ---------------------------------------------------------------------------
+// getRuntimeNewProjectCommand (ADR-1239 Phase B / #1679 AC2) — the per-runtime
+// /gsd-new-project invocation syntax for the post-install next-step message.
+// ---------------------------------------------------------------------------
+
+const GOLDEN_COMMAND_MAP = {
+  // 4 real overrides (the rest use the default):
+  gemini: '/gsd:new-project',
+  codex: '$gsd-new-project',
+  cursor: 'gsd-new-project (mention the skill name)',
+  kimi: '/skill:gsd-new-project',
+};
+const DEFAULT_CMD = '/gsd-new-project';
+
+test('getRuntimeNewProjectCommand: the 4 overrides + the default for the other 12 runtimes', () => {
+  for (const [id, expected] of Object.entries(GOLDEN_COMMAND_MAP)) {
+    assert.strictEqual(getRuntimeLabel ? getRuntimeNewProjectCommand(id) : null, expected, `override ${id}`);
+  }
+  // sanity: getRuntimeNewProjectCommand is imported alongside getRuntimeLabel above
+});
+
+test('getRuntimeNewProjectCommand: claude/unknown/empty + the 12 non-override runtimes → default', () => {
+  for (const id of ['claude', 'opencode', 'kilo', 'copilot', 'antigravity', 'windsurf', 'augment', 'trae', 'cline', 'qwen', 'hermes', 'codebuddy', 'unknown', '']) {
+    assert.strictEqual(getRuntimeNewProjectCommand(id), DEFAULT_CMD, `runtime '${id}' must return the default command`);
+  }
+});