skills: Add /to-spec skill.

.claude/skills/to-spec/SKILL.md

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+---
+name: to-spec
+description: Turn the current conversation into a spec and write it to .claude/spec/ — no interview, just synthesis of what you've already discussed.
+disable-model-invocation: true
+---
+
+This skill takes the current conversation context and codebase understanding and produces a spec. Do NOT interview the user — just synthesize what you already know.
+
+If the conversation doesn't actually contain a feature or problem to synthesize a spec from, say so and ask what it's for instead of fabricating one.
+
+## Process
+
+1. Explore the repo until you can name the existing modules, flows, and seams the feature will touch, if you haven't already. Use the project's domain glossary vocabulary throughout the spec, and respect any ADRs in the area you're touching.
+
+2. Sketch out the seams at which you're going to test the feature. Existing seams should be preferred to new ones. Use the highest seam possible. If new seams are needed, propose them at the highest point you can. The fewer seams across the codebase, the better - the ideal number is one.
+
+Check with the user that these seams match their expectations.
+
+3. Derive a short kebab-case feature-slug from the feature's name (e.g. `checkout-flow`). Tell the user the path you're about to write to (`.claude/spec/<feature-slug>.md`). If a file already exists there, summarize what would change and confirm with the user before overwriting it — never overwrite silently.
+
+4. Write the spec using the format in [SPEC-FORMAT.md](./SPEC-FORMAT.md) to `.claude/spec/<feature-slug>.md`, creating the `.claude/spec/` directory if it doesn't exist yet.

.claude/skills/to-spec/SPEC-FORMAT.md

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+# Spec Format
+
+## Template
+
+```md
+## Problem Statement
+
+The problem that the user is facing, from the user's perspective.
+
+## Solution
+
+The solution to the problem, from the user's perspective.
+
+## User Stories
+
+An extensive, numbered list of user stories, covering all aspects of the feature. Each user story should be in the format of:
+
+1. As an <actor>, I want a <feature>, so that <benefit>
+
+<user-story-example>
+1. As a mobile bank customer, I want to see balance on my accounts, so that I can make better informed decisions about my spending
+</user-story-example>
+
+## Implementation Decisions
+
+A list of implementation decisions that were made. This can include:
+
+- The modules that will be built/modified
+- The interfaces of those modules that will be modified
+- Technical clarifications from the developer
+- Architectural decisions
+- Schema changes
+- API contracts
+- Specific interactions
+
+## Testing Decisions
+
+A list of testing decisions that were made. Include:
+
+- A description of what makes a good test (only test external behavior, not implementation details)
+- Which modules will be tested
+- Prior art for the tests (i.e. similar types of tests in the codebase)
+
+## Out of Scope
+
+A description of the things that are out of scope for this spec.
+
+## Further Notes
+
+Any further notes about the feature.
+```
+
+## Rules
+
+- **Don't include specific file paths or code snippets.** They may end up being outdated very quickly. Exception: if a prototype produced a snippet that encodes a decision more precisely than prose can (state machine, reducer, schema, type shape), inline it within the relevant decision in Implementation Decisions and note briefly that it came from a prototype. Trim to the decision-rich parts — not a working demo, just the important bits.