docs: diagram + multi-format documentation across README, make-pdf skill, and how-to guide

README gains /make-pdf (Publisher) and /diagram (Diagram Maker) rows in the
sprint table. make-pdf's skill doc — the agent-facing contract — gains Core
patterns for mermaid/excalidraw fences (title/render=false/page= options),
the image policy ({width=}/{page=} directives, zero-truncation, conservative
auto-landscape), --to html|docx, and --strict, plus the --to vs --format
disambiguation in Common flags. New docs/howto-diagrams-and-formats.md is
the user-facing walkthrough: fences, directives, formats, /diagram triplet,
the mermaid racetrack trick, troubleshooting.

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

README.md

@@ -206,6 +206,8 @@ Each skill feeds into the next. `/office-hours` writes a design doc that `/plan-
 | `/autoplan` | **Review Pipeline** | One command, fully reviewed plan. Runs CEO → design → eng review automatically with encoded decision principles. Surfaces only taste decisions for your approval. |
 | `/spec` | **Spec Author** | Turn vague intent into a precise, executable spec in five phases (why, scope, technical with mandatory code-reading, draft, file). Codex quality gate before file (blocks below 7/10), fail-closed secret redaction, dedupe against existing issues, archive to `$GSTACK_STATE_ROOT/projects/$SLUG/specs/` for team-corpus recall. `--execute` spawns `claude -p` in a fresh worktree; `/ship` auto-closes the source issue on merge. Plan-mode aware. |
 | `/learn` | **Memory** | Manage what gstack learned across sessions. Review, search, prune, and export project-specific patterns, pitfalls, and preferences. Learnings compound across sessions so gstack gets smarter on your codebase over time. |
+| `/make-pdf` | **Publisher** | Markdown in, publication-quality document out. Mermaid and excalidraw fences render as vector diagrams, fully offline. Images scale to the page and never truncate; wide diagrams get their own landscape page. `--to html` emits one self-contained file, `--to docx` a Word doc. |
+| `/diagram` | **Diagram Maker** | English in, editable diagram out. Emits a triplet: mermaid source, `.excalidraw` you can open and edit on excalidraw.com, and rendered SVG/PNG. Hand-drawn aesthetic, zero network. Embed the source in markdown and `/make-pdf` renders it. |
 
 ### Which review should I use?