feat(#785): write .cursor/commands/ Cursor 1.6 slash-command surface (#805)

* feat(#785): write .cursor/commands/ as Cursor 1.6 slash-command surface

Cursor 1.6 (released 2025-09-12) introduced plain-markdown slash commands
in `.cursor/commands/<name>.md` — no frontmatter, invocable via `/` in the
Agent input. GSD previously emitted only `~/.cursor/skills/` for Cursor.

This PR wires a second artifact kind for `cursor` in
`runtime-artifact-layout.cts`: `convertedCommandsKind('commands', 'gsd-',
'convertClaudeCommandToCursorCommand', configDir)`. The new kind applies the
same `convertClaudeToCursorMarkdown` transforms (tool renames, brand
substitution, slash-command normalisation) and then strips YAML frontmatter
so the output is plain prose. Skills output is unchanged.

`stageCommandsForRuntimeFlat` in `install-profiles.cts` stages each source
`.md` as a flat `<stem>.md` in a temp dir; the existing `_copyStaged` commands
path then prefixes and copies to `<configDir>/commands/`.

`.cursor/mcp.json` is explicitly OUT OF SCOPE: GSD ships no MCP server; the
`mcpServers` schema cannot be usefully populated by the installer.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* refactor(#785): address review nit

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/cursor-conversion.test.cjs

@@ -4,6 +4,9 @@
  * Ensures Cursor frontmatter names are emitted as plain identifiers
  * (without surrounding quotes), so Cursor does not treat quotes as
  * literal parts of skill/subagent names.
+ *
+ * Also covers convertClaudeCommandToCursorCommand (#785 — Cursor 1.6
+ * slash commands via .cursor/commands/).
  */
 
 process.env.GSD_TEST_MODE = '1';
@@ -14,6 +17,7 @@ const assert = require('node:assert/strict');
 const {
   convertClaudeCommandToCursorSkill,
   convertClaudeAgentToCursorAgent,
+  convertClaudeCommandToCursorCommand,
 } = require('../bin/install.js');
 
 describe('convertClaudeCommandToCursorSkill', () => {
@@ -79,3 +83,71 @@ Planner body
     assert.ok(!result.includes('name: "gsd-planner"'), 'quoted agent name is not emitted');
   });
 });
+
+// ─── convertClaudeCommandToCursorCommand (#785) ───────────────────────────────
+
+describe('convertClaudeCommandToCursorCommand (#785 — Cursor 1.6 .cursor/commands/)', () => {
+  test('strips YAML frontmatter — output is plain markdown', () => {
+    const input = `---
+name: help
+description: Show help for GSD commands
+---
+
+# GSD Help
+
+Use \`/gsd-help\` to see available commands.
+`;
+
+    const result = convertClaudeCommandToCursorCommand(input);
+    assert.ok(!result.startsWith('---'), 'cursor commands must not have YAML frontmatter');
+    assert.ok(!result.includes('name: help'), 'name field must be stripped');
+    assert.ok(!result.includes('description:'), 'description field must be stripped');
+    assert.ok(result.includes('GSD Help'), 'body content must be preserved');
+  });
+
+  test('applies convertClaudeToCursorMarkdown transforms (Bash → Shell, Claude Code → Cursor)', () => {
+    const input = `---
+name: quick
+description: Quick task
+---
+
+Use Bash( to run commands.
+This runs in Claude Code.
+`;
+
+    const result = convertClaudeCommandToCursorCommand(input);
+    assert.ok(result.includes('Shell('), 'Bash( should be renamed to Shell(');
+    assert.ok(!result.includes('Claude Code'), 'Claude Code brand reference should be replaced');
+    assert.ok(result.includes('Cursor'), 'should reference Cursor instead');
+  });
+
+  test('normalizes gsd: colon slash commands to gsd- hyphen form', () => {
+    const input = `---
+name: plan-phase
+description: Plan a phase
+---
+
+Next step: /gsd:execute-phase 17
+`;
+
+    const result = convertClaudeCommandToCursorCommand(input);
+    assert.ok(result.includes('/gsd-execute-phase 17'), 'colon form should become hyphen form');
+    assert.ok(!result.includes('/gsd:execute-phase'), 'colon form should be removed');
+  });
+
+  test('handles input with no frontmatter gracefully', () => {
+    const input = `# No Frontmatter Command
+
+Some body content.
+`;
+
+    const result = convertClaudeCommandToCursorCommand(input);
+    assert.ok(!result.startsWith('---'), 'output must not start with ---');
+    assert.ok(result.includes('No Frontmatter Command'), 'body should be preserved');
+  });
+
+  test('is exported from install.js', () => {
+    assert.strictEqual(typeof convertClaudeCommandToCursorCommand, 'function',
+      'convertClaudeCommandToCursorCommand must be exported from install.js');
+  });
+});