chore: add fallow + clean up dead code and duplication

[andrelandgraf]

Summary

Wire up fallow as a dev dependency and document a workflow in AGENTS.md for running npx fallow dead-code and npx fallow dupes after every change, with first-principles guidance on when to remove vs leverage flagged code and when duplication is worth unifying vs leaving alone.

Then ran fallow across the repo and acted only on the findings that genuinely made the codebase clearer.

Dead code removed

• Delete unused custom components: pillar-strip.tsx, appkit-version-picker.tsx, selected-items.tsx, recipe-list.tsx, copy-button.tsx (verified zero references).
• Delete dead exports/declarations: getRecipeMarkdownApiPath, COOKBOOK_FILES/CookbookFile, pillars/Pillar, SolutionTag alias, unused useAllRawRecipeMarkdown/useRecipeSections/useRawExampleMarkdown hooks, buildRobotsTxt re-export, dead re-exports in content-markdown.ts.
• Demote internal-only types to module-private (no behavior change).
• Drop transitive-only deps date-fns and @hookform/resolvers.
• Update author-recipes-and-cookbooks skill to match the actual hooks.

Duplication unified (real same-logic-twice cases)

• New useAgentMarkdown hook consolidates the agent-prompt builder shared by ai-export-menu and copy-prompt-button.
• New useCookbookMarkdown hook removes ~25 lines of boilerplate from each of the 5 cookbook pages under src/pages/templates/.
• New BootstrapCopyButton replaces the duplicated copy-state machines in hero-section and wizard-flow (also resolves a CopyPromptButton name collision).
• Extract buildIncludedTemplateLinks and validateContentFolder helpers for duplicated loops in build-example-markdown.ts and scripts/validate-content.mjs.

Duplication left alone (intentional repetition for clarity)

• Test files where independent flows happen to share shape.
• Cross-callsite filter patterns where each consumer reads the catalog differently.
• Tiny 5-line handler boilerplate where extracting a helper saves nothing.

Configuration

.fallowrc.json scopes future runs by ignoring examples/** and content/**, plus documented false positives (mcp-handler peer dep, Docusaurus type wiring, transitive remark deps, the mcp-handler HTTP method exports, and the shadcn UI kit catalog).

Results

Check	Before	After
Unused exports (actionable)	33	0
Unused type exports (actionable)	9	0
Unused dependencies (actionable)	3	0
Duplicated lines	5,171 (14.9%)	478 (2.2%)

Net diff: -1,091 lines (-1,396 / +305) across 36 files.

Test plan

• npm run fmt
• npm run typecheck
• npm run validate:content
• npm run verify:images
• npm run build
• npm run test:smoke — 165 tests pass
• npx fallow dead-code — 0 actionable findings
• npx fallow dupes — only intentional / accepted patterns remain

[andrelandgraf]

Thanks @thisistonydang for tweeting about fallow ;)