refactor: switch index from SQLite to JSON with relative paths

manage_index.py:
- Complete rewrite from SQLite to JSON file format
- All file paths stored as relative (relative to video directory)
- Moving the entire video directory does not break references
- Paths outside video dir stored as absolute (editable in text editor)
- Same 8 subcommands, same CLI interface

SKILL.md:
- Step 5: SQLite schema replaced with JSON structure example
- Verification checklist: .vedit.db → .vedit.json

README.md + README.zh.md:
- Features: 'SQLite database' → 'JSON file with relative paths'
- Project structure: 'SQLite index management' → 'JSON index management'
- 'Index Database' section → 'Index File' with relative path explanation

.gitignore:
- *.vedit.db → *.vedit.json

.gitignore

@@ -17,8 +17,8 @@ Thumbs.db
 *.swo
 *~
 
-# Index databases (runtime artifacts, not source)
-*.vedit.db
+# Index files (runtime artifacts, not source)
+*.vedit.json
 
 # Extracted clips and frames (runtime artifacts)
 *_clips/