feat: add flutter-deck-author agent skill (Markdown outline → slides)

[mkobuolys]

What

Adds a sixth agent skill, flutter-deck-author, that turns a Markdown outline into idiomatic flutter_deck slide files.

The existing five skills are all reference skills (they teach how to use the API). None help a user go from a topic/outline to a finished deck — the actual authoring bottleneck. This skill closes that gap: the author writes content in Markdown, the agent emits one Dart file per slide.

It is content-only and composes with the existing skills:

• Defers app scaffolding to flutter-deck-presentation-setup.
• Refers to flutter-deck-slides / flutter-deck-theming for factory and styling detail.
• Does not add the dependency, wire FlutterDeckApp, or define themes.

How it works

• Segments Markdown into slides (# → opening title, --- hard break, else each ##).
• Maps each block to a built-in factory by content shape (blockquote → quote, short stat → bigFact, fenced code → blank + FlutterDeckCodeHighlight, image → image, text+media → split, heading + bullets → blank + FlutterDeckBulletList), with a stated top-to-bottom precedence rule.
• Override directives (invisible HTML comments): <!-- slide: <factory> -->, <!-- steps: reveal -->, <!-- notes: … -->, <!-- route: /custom -->.
• Derives route, class name, file name, and nav-drawer title: from each heading (with a digit-leading fallback, e.g. 100% → Slide100, /100).
• Wires the lib/slides/ barrel (alphabetical) and the FlutterDeckApp.slides list (outline order).

Registered in README.md and the docs site (doc/website/source/ai/agent-skills.md).

Verification

• The skill's worked example was built into a throwaway deck and passes flutter analyze + flutter build web; the two embedded slide examples are byte-for-byte identical to that verified fixture.
• Every API used was cross-checked against the package source.
• A cold-follow test — a fresh agent given only this SKILL.md and the worked-example outline — reproduced an identical file set: same routes, class names, factory choices, steps, and barrel (no factory/route/class divergence).

Notes

• .gitignore now ignores docs/superpowers/ (local design/plan scratch).
• No Dart source in the package changed — docs/skill only.

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