Add dry-run index preview - #274

Summary

ccc index --dry now lists files that would enter or leave the index source set without running the indexer. The command compares files matched from the project tree with persisted process_file component paths, then prints sorted additions and deletions with exact counts.

Dry runs require an initialized project and do not auto-create settings. They open the CocoIndex application database through its read-only inspection API and leave the indexed paths unchanged.

Preview flow

flowchart LR
    Command["ccc index --dry"] --> Settings["Load project settings"]
    Settings --> Matched["Walk matched source files"]
    Command --> Persisted["Read persisted process_file paths"]
    Matched --> Compare["Compare path sets"]
    Persisted --> Compare
    Compare --> Added["Print sorted additions"]
    Compare --> Deleted["Print sorted deletions"]

The preview reads source and index state but does not start the indexing flow.

Matching behavior

• Use the same include patterns, exclude patterns, nested .gitignore rules, and max_file_size limit as indexing.
• Report matched empty files even when they produce no vector rows, because source membership is tracked by persisted components.
• Treat every matched file as an addition when no index database exists.
• Share the CocoIndex application name between indexing and inspection so the persisted paths are read from the correct application.

Testing

• Focused dry-run tests: 4 passed.
• The exact-path prek gate passed trailing whitespace, end-of-file, large-file, conflict, gitleaks, Ruff lint, Ruff format, mypy, and the full pytest suite.
• git diff --check upstream/main...HEAD passed.