feat(diagram): /diagram skill — English in, editable diagram triplet out

New skill: agent authors mermaid from the user's description and renders the
triplet through the offline diagram-render bundle in the browse daemon —
.mmd source (the single source of truth), editable .excalidraw (opens at
excalidraw.com, round-trips back through re-render), and SVG + PNG. Flowcharts
convert to fully editable scenes; other mermaid types render with an explicit
upstream-converter limitation note. Never ships an unrendered source file;
offline is the contract (no CDN fallback). Inventory rows in AGENTS.md +
docs/skills.md; generated SKILL.md + llms.txt via gen:skill-docs.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

gstack/llms.txt

@@ -26,6 +26,7 @@ Conventions:
 - [/design-review](design-review/SKILL.md): Designer's eye QA: finds visual inconsistency, spacing issues, hierarchy problems, AI slop patterns, and slow interactions — then fixes them.
 - [/design-shotgun](design-shotgun/SKILL.md): Design shotgun: generate multiple AI design variants, open a comparison board, collect structured feedback, and iterate.
 - [/devex-review](devex-review/SKILL.md): Live developer experience audit.
+- [/diagram](diagram/SKILL.md): Turn an English description (or mermaid source) into a diagram triplet: the source, an editable .excalidraw file you can open on excalidraw.com, and rendered SVG + PNG.
 - [/document-generate](document-generate/SKILL.md): Generate missing documentation from scratch for a feature, module, or entire project.
 - [/document-release](document-release/SKILL.md): Post-ship documentation update.
 - [/freeze](freeze/SKILL.md): Restrict file edits to a specific directory for the session.