feat(gstack): generate llms.txt — single-file capability index for AI agents

Adds scripts/gen-llms-txt.ts: produces gstack/llms.txt at repo root,
indexing every skill (47), every browse command (75), and design
commands when the design CLI is present. Per the llmstxt.org
convention, agents can read one file to learn what gstack offers
instead of crawling 47 SKILL.md files.

Sources:
- skill SKILL.md.tmpl frontmatter (name + description block scalar)
- browse/src/commands.ts COMMAND_DESCRIPTIONS (sorted by category)
- design/src/commands.ts COMMAND_DESCRIPTIONS if present (best-effort)

Wired into scripts/gen-skill-docs.ts as a post-step so it regenerates
on every `bun run gen:skill-docs` (the same script that re-emits all
SKILL.md files). Failures are non-fatal warnings, not build breaks —
the generator never blocks SKILL.md regen.

Strict mode (--strict, also used by tests) throws when a skill is
missing name or description in its frontmatter, catching missing
metadata before it ships.

Tests: shape (top-level sections, sort order, single-line summary
discipline), every-skill-and-command-appears, strict-mode rejection of
incomplete frontmatter, and freshness check that the committed
gstack/llms.txt matches what the generator produces now.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/gen-skill-docs.ts

@@ -662,3 +662,21 @@ if (!DRY_RUN) {
     }
   } catch { /* non-fatal */ }
 }
+
+// Regenerate gstack/llms.txt — single-file capability index for AI agents.
+// Runs after SKILL.md generation so it sees current skill descriptions and
+// browse command list. Freshness is asserted in test/llms-txt-shape.test.ts.
+if (!DRY_RUN) {
+  try {
+    const { writeLlmsTxt } = await import('./gen-llms-txt');
+    const result = await writeLlmsTxt();
+    if (result.warnings.length > 0) {
+      for (const w of result.warnings) console.error(`[gen-llms-txt] WARN: ${w}`);
+    } else {
+      console.log(`[gen-llms-txt] gstack/llms.txt: ${result.skills.length} skills, ${result.browseCommands.length} browse commands`);
+    }
+  } catch (err) {
+    const msg = err instanceof Error ? err.message : String(err);
+    console.error(`[gen-llms-txt] FAILED: ${msg}`);
+  }
+}