fix(explore): search file content instead of filename for similar code

[prosdev]

🐛 Bug

explore similar was searching the filename as text instead of reading and searching the file's content embeddings.

$ dev explore similar coordinator.ts
⚠ No similar code found  # Searched "coordinator.ts" as text ❌

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✅ Fix

Read file content and search its embeddings for semantic similarity.

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📝 Changes

1. Extracted File Utilities (cli/src/utils/file.ts)

Pure, testable functions:

• resolveFilePath: Resolve paths relative to repo root
• normalizeFilePath: Create relative paths
• readFileContent: Read and validate file content
• prepareFileForSearch: Prepare file for similarity search

2. 100% Test Coverage (cli/src/utils/file.test.ts)

22 unit tests covering:

• Path resolution and normalization
• File reading with validation
• Error handling (missing, empty files)
• Edge cases (special chars, large files, nested dirs)

3. Refactored explore similar

• Uses new utilities for clean separation
• Added --threshold option (default: 0.5)
• Better error messages

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🧪 Verification

Before:

$ dev explore similar packages/core/src/vector/store.ts
⚠ No similar code found

After:

$ dev explore similar packages/core/src/vector/store.ts
✔ Found 3 similar files

1. packages/core/src/vector/README.md (64.1% similar)
2. packages/core/src/vector/embedder.ts (61.7% similar)  
3. packages/core/src/scanner/README.md (56.8% similar)

Tests:

$ pnpm test packages/cli/src/utils/file.test.ts
✓ 22 tests passing ✅

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📊 Stats

• +329 lines, -4 lines
• 3 files changed
• 22 new tests (100% coverage on utilities)
• Follows testability philosophy (extract pure functions, test them)

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🚀 Impact

Users can now find semantically similar code based on actual file content, not filenames. Feature works as intended!