feat(prototype): add throwaway prototype skill

skills/prototype/SKILL.md

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+---
+name: prototype
+description: Build a throwaway prototype to answer a design question. Use when the user wants to sanity-check whether a state model or logic feels right, or explore what a UI should look like.
+---
+
+# Prototype
+
+A prototype is **throwaway code that answers a question**. The question decides the shape.
+
+## Pick a branch
+
+Identify which question is being answered — from the user's prompt, the surrounding code, or by asking if the user is around:
+
+- **"Does this logic / state model feel right?"** → [LOGIC.md](LOGIC.md). Build a tiny interactive terminal app that pushes the state machine through cases that are hard to reason about on paper.
+- **"What should this look like?"** → [UI.md](UI.md). Generate several radically different UI variations on a single route, switchable via a URL search param and a floating bottom bar.
+
+The two branches produce very different artifacts — getting this wrong wastes the whole prototype. If the question is genuinely ambiguous and the user isn't reachable, default to whichever branch better matches the surrounding code (a backend module → logic; a page or component → UI) and state the assumption at the top of the prototype.
+
+## Rules that apply to both
+
+1. **Throwaway from day one, and clearly marked as such.** Locate the prototype code close to where it will actually be used (next to the module or page it's prototyping for) so context is obvious — but name it so a casual reader can see it's a prototype, not production. For throwaway UI routes, obey whatever routing convention the project already uses; don't invent a new top-level structure.
+2. **One command to run.** Whatever the project's existing task runner supports — `pnpm <name>`, `python <path>`, `bun <path>`, etc. The user must be able to start it without thinking.
+3. **No persistence by default.** State lives in memory. Persistence is the thing the prototype is _checking_, not something it should depend on. If the question explicitly involves a database, hit a scratch DB or a local file with a clear "PROTOTYPE — wipe me" name.
+4. **Skip the polish.** No tests, no error handling beyond what makes the prototype _runnable_, no abstractions. The point is to learn something fast.
+5. **Surface the state.** After every action (logic) or on every variant switch (UI), print or render the full relevant state so the user can see what changed.
+6. **Capture it when done.** When the caller permits implementation, fold any validated decision into the real code. A planning-only caller such as Wayfinder stops at the verdict. Commit the prototype itself to a throwaway branch, out of main, as a **primary source**. Write a Prototype artifact under `$(xdg-user-dir DOCUMENTS)/ai-artifacts/projects/<project>/`, where `<project>` is the lowercase basename of the current working directory, following the vault's `AGENTS.md`. Include the question, a context pointer to the branch, run instructions, and the verdict, plus screenshots and useful code snippets where they help preserve the result. The main branch keeps only a validated decision that the caller permitted the skill to fold in.