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>

src/runtime-artifact-layout.cts

@@ -21,6 +21,7 @@ const {
   stageSkillsForProfile,
   stageAgentsForProfile,
   stageSkillsForRuntimeAsSkills,
+  stageCommandsForRuntimeFlat,
 } = installProfiles;
 
 // In .cts (CommonJS output) files, `require` is available as a global.
@@ -225,6 +226,40 @@ function skillsKind(
   };
 }
 
+/**
+ * Build a converted-commands kind descriptor for runtimes that use a flat
+ * commands directory with per-file conversion (e.g. Cursor 1.6 slash commands).
+ *
+ * Unlike `commandsKind` (which passes raw source files through), this kind
+ * applies `converterName` from bin/install.js exports to each file during
+ * staging, writing flat `${prefix}${stem}.md` files to the staged directory.
+ *
+ * The staged files are then written by `_copyStaged` (commands branch) which
+ * handles prefix logic via the existing layout machinery.
+ *
+ * @param destSubpath   destination subpath within configDir (e.g. 'commands')
+ * @param prefix        filename prefix, e.g. 'gsd-'
+ * @param converterName name of converter function in bin/install.js exports
+ * @param configDir     runtime config dir (for .gsd-source marker resolution)
+ */
+function convertedCommandsKind(
+  destSubpath: string,
+  prefix: string,
+  converterName: string,
+  configDir: string,
+): ArtifactKind {
+  return {
+    kind: 'commands',
+    destSubpath,
+    prefix,
+    stage: (resolved) => {
+      const installExports = getInstallExports();
+      const converter = installExports[converterName] as (content: string, commandName: string) => string;
+      return stageCommandsForRuntimeFlat(findInstallSourceRoot(configDir), resolved, converter, prefix);
+    },
+  };
+}
+
 // ---------------------------------------------------------------------------
 // Public API
 // ---------------------------------------------------------------------------
@@ -257,7 +292,14 @@ function resolveRuntimeArtifactLayout(runtime: string, configDir: string, scope:
       break;
 
     case 'cursor':
-      kinds = [skillsKind('skills', 'gsd-', 'convertClaudeCommandToCursorSkill', 'cursor', configDir)];
+      // Cursor 1.6+ supports two artifact surfaces:
+      //   1. skills/gsd-<name>/SKILL.md  — rich skills with frontmatter + adapter header
+      //   2. commands/gsd-<name>.md      — plain markdown slash commands (no frontmatter)
+      //      accessed via '/' in the Agent input (#785)
+      kinds = [
+        skillsKind('skills', 'gsd-', 'convertClaudeCommandToCursorSkill', 'cursor', configDir),
+        convertedCommandsKind('commands', 'gsd-', 'convertClaudeCommandToCursorCommand', configDir),
+      ];
       break;
 
     case 'gemini':