add Claude Code automations: hooks, skills, and subagent

- Hooks: auto build check on edit, protect test resource files
- Skills: test-manglings, sync-with-swift
- Subagent: file-size-checker for 500-line limit

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: oozoofrog

.claude/agents/file-size-checker.md

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+# File Size Checker
+
+Check all Swift source files in `Sources/` for the 500-line limit.
+
+## Instructions
+
+1. Count lines in all `.swift` files under `Sources/`
+2. Report any files exceeding 500 lines
+3. For each oversized file, suggest logical split points based on:
+   - Protocol conformances
+   - Extension boundaries
+   - Logical groupings of related methods
+4. Focus on files that were recently modified (check `git diff --name-only`)

.claude/settings.json

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+{
+  "hooks": {
+    "PostToolUse": [
+      {
+        "matcher": "Edit|Write",
+        "command": "swift build 2>&1 | tail -20"
+      }
+    ],
+    "PreToolUse": [
+      {
+        "matcher": "Edit|Write",
+        "command": "case \"$CLAUDE_FILE_PATH\" in */Resources/manglings*) echo 'BLOCKED: test resource file'; exit 1;; esac"
+      }
+    ]
+  }
+}

.claude/skills/sync-with-swift/SKILL.md

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+---
+name: sync-with-swift
+description: Guide for syncing with upstream Swift compiler demangling changes
+---
+
+# sync-with-swift
+
+This project is a pure Swift port of the Swift compiler's `swift-demangle`. When syncing with a new Swift version:
+
+## Checklist
+
+1. **Node.Kind**: Check for new enum cases in `Node.swift` `Kind` enum
+2. **Demangler**: Check for new operators in `demangleOperator()` in `Demangler.swift`
+3. **NodePrinter**: Add corresponding print rules in `printNode()` in `NodePrinter.swift`
+4. **DemangleOptions**: Check for new options in `DemangleOptions.swift`
+5. **Test data**: Update `Tests/SwiftDemangleTests/Resources/manglings.txt` with new test cases
+
+## Reference
+
+- Upstream source: https://github.com/swiftlang/swift/tree/main/lib/Demangling
+- Diff guide: `Diff.md` in project root tracks differences from upstream

.claude/skills/test-manglings/SKILL.md

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+---
+name: test-manglings
+description: Validate mangled symbols against expected demangled output
+disable-model-invocation: true
+---
+
+# test-manglings
+
+Given a mangled Swift symbol:
+
+1. Check if it exists in `Tests/SwiftDemangleTests/Resources/manglings.txt`
+2. Run `swift test --filter testManglings` to verify demangling results
+3. Report pass/fail with the expected vs actual demangled output
+
+If a new symbol pair is provided (mangled ---> demangled), add it to the appropriate resource file and run the test to confirm.