Add Claude Code GitHub Workflow

.github/workflows/claude-code-review.yml

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+name: Claude Code Review
+on:
+  pull_request:
+    types: [opened, synchronize]
+jobs:
+  claude-review:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      pull-requests: read
+      issues: read
+      id-token: write
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v5
+        with:
+          fetch-depth: 1
+      - name: Run Claude Code Review
+        id: claude-review
+        uses: anthropics/claude-code-action@v1
+        with:
+          claude_code_oauth_token: ${{ secrets.CLAUDE_CODE_OAUTH_TOKEN }}
+          prompt: |
+            REPO: ${{ github.repository }}
+            PR NUMBER: ${{ github.event.pull_request.number }}
+
+            Please review this pull request and provide feedback on:
+            - Code quality and best practices
+            - Potential bugs or issues
+            - Performance considerations
+            - Security concerns
+            - Test coverage
+
+            Use the repository's CLAUDE.md for guidance on style and conventions. Be constructive and helpful in your feedback.
+
+            Use `gh pr comment` with your Bash tool to leave your review as a comment on the PR.
+
+          # See https://github.com/anthropics/claude-code-action/blob/main/docs/usage.md
+          # or https://docs.claude.com/en/docs/claude-code/sdk#command-line for available options
+          claude_args: '--allowed-tools "Bash(gh issue view:*),Bash(gh search:*),Bash(gh issue list:*),Bash(gh pr comment:*),Bash(gh pr diff:*),Bash(gh pr view:*),Bash(gh pr list:*)"'

.github/workflows/claude.yml

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+name: Claude Code
+on:
+  issue_comment:
+    types: [created]
+  pull_request_review_comment:
+    types: [created]
+  issues:
+    types: [opened, assigned]
+  pull_request_review:
+    types: [submitted]
+jobs:
+  claude:
+    if: |
+      (github.event_name == 'issue_comment' && contains(github.event.comment.body, '@claude')) ||
+      (github.event_name == 'pull_request_review_comment' && contains(github.event.comment.body, '@claude')) ||
+      (github.event_name == 'pull_request_review' && contains(github.event.review.body, '@claude')) ||
+      (github.event_name == 'issues' && (contains(github.event.issue.body, '@claude') || contains(github.event.issue.title, '@claude')))
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      pull-requests: read
+      issues: read
+      id-token: write
+      actions: read # Required for Claude to read CI results on PRs
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 1
+      - name: Run Claude Code
+        id: claude
+        uses: anthropics/claude-code-action@v1
+        with:
+          claude_code_oauth_token: ${{ secrets.CLAUDE_CODE_OAUTH_TOKEN }}
+          additional_permissions: |
+            actions: read

CLAUDE.md

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+# type-fest
+
+TypeScript utility types library. Pure type-level programming—no runtime code.
+
+**Read `.github/contributing.md` for complete guidelines.**
+
+## Commands
+
+```bash
+npm test         # MUST pass before committing (runs all below)
+npm run test:tsc # TypeScript compiler
+npm run test:tsd # Type tests (tsd)
+npm run test:xo  # Linter
+```
+
+## File Structure
+
+```
+source/type-name.d.ts → Type definition (.d.ts REQUIRED)
+test-d/type-name.ts   → Tests (tsd)
+index.d.ts            → Export (MUST add, lint enforces)
+readme.md             → API docs (add to category)
+source/internal/      → Shared helpers (not exported)
+```
+
+## Code Patterns
+
+```ts
+// ✅ CORRECT
+import type {IsNever} from './is-never.d.ts';  // Include .d.ts
+export type MyType<Value extends string> = ... // Descriptive names (not T/U)
+export {};                                      // REQUIRED at end
+
+// ❌ WRONG
+import type {IsNever} from './is-never';       // Missing .d.ts
+export type MyType<T> = ...;                   // Single-letter
+
+/**
+Creates a tuple type of specified length with elements of specified type.
+
+Use-cases:
+- Define fixed-length arrays with specific types
+
+@example
+```
+import type {TupleOf} from 'type-fest';
+type RGB = TupleOf<3, number>; //=> [number, number, number]
+```
+
+@category Array
+*/
+export type TupleOf<Length extends number, Fill = unknown> = ...;
+export {};
+```
+
+Type params: `Value`, `Target`, `Item`, `Length`, `Element`, `Key` (not `T`, `U`, `V`, `K`)
+Docs: No `*` prefix. First line → readme. Include use-cases, examples, `@category`.
+
+## Testing
+
+**ALWAYS test edge cases** (`any`, `never`, `unknown`).
+
+```ts
+expectType<string>('' as MyType<'foo'>);           // Basic
+expectType<any>('' as MyType<any>);                // any/never/unknown
+expectNotAssignable<MyType<'foo'>>({wrong: true}); // Negative
+expectError<MyType<number>>();                     // Should error
+```
+
+Study: `source/tuple-of.d.ts`, `is-equal.d.ts`, `literal-union.d.ts`, `internal/*.d.ts`
+
+## Workflow
+
+1. Research - Study `source/` similar types + online research
+2. Define - `source/type-name.d.ts` with docs
+3. Test - `test-d/type-name.ts` with edge cases
+4. Export - Add to `index.d.ts` + readme.md
+5. Verify - `npm test` must pass
+6. Commit - ``Add `TypeName` type`` or ``  `TypeName`: Fix description``
+
+## Critical Rules
+
+1. Import paths MUST include `.d.ts` extension
+2. Files MUST end with `export {};`
+3. Types MUST be exported from `index.d.ts`
+4. Type params MUST use descriptive names (not `T`/`U`)
+5. MUST test `any`, `never`, `unknown` edge cases
+6. First doc line MUST be concise (goes in readme)
+7. Use realistic examples with `//=>` comments
+8. Include `@category` tags
+9. Fix lint issues, don't disable rules
+
+## Type Programming
+
+- Conditionals: `T extends U ? X : Y`
+- Recursion: `type Loop<T, Acc> = ... Loop<...> ...`
+- Extract: `infer Item`
+- Distribute: `T extends any ? ... : never`
+- Count via tuple `['length']`
+- Union distribution is tricky—test it
+
+## Philosophy
+
+- Correctness > cleverness
+- Real problems only
+- Docs teach how, not what
+- Edge cases mandatory
+- One concept per PR
+- Descriptive names > brevity
+
+## Troubleshooting
+
+- Tests fail? Check `.d.ts` imports, `export {};`, edge cases (`any`, `never`, `unknown`)
+- Lint errors? Add to `index.d.ts`, fix issues (don't disable rules)