Refactor: Centralize manifest and blob management operations - #4

This refactoring consolidates duplicate manifest and blob management logic
into reusable modules within socket-patch, establishing it as the single
source of truth for these operations.

## Changes

### New Modules
- **src/constants.ts**: Standardized path constants (DEFAULT_BLOB_FOLDER, etc.)
- **src/manifest/operations.ts**: Core manifest utilities
  - getReferencedBlobs(): Extract all blob hashes from a manifest
  - diffManifests(): Calculate manifest differences
  - validateManifest(): Manifest validation
  - readManifest() / writeManifest(): Filesystem operations
- **src/manifest/recovery.ts**: Robust manifest recovery with pluggable callbacks
  - Automatic repair of invalid patches
  - Dependency-agnostic design with event-based logging
  - Refetch function for external patch sources

### Updated Files
- **src/utils/cleanup-blobs.ts**: Now uses getReferencedBlobs() utility
- **src/patch/apply.ts**: Removed unused _pkgName parameter
- **src/index.ts**: Export new manifest modules
- **package.json**: Added exports for new modules

## Benefits
- Eliminates ~200 lines of duplicate code
- Single source of truth for manifest operations
- Better testability with dependency injection
- Reusable across CLI and service contexts
- Standardized constants and utilities

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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