"--dry-run" flag for generation paths

[LeeSinLiang]

--dry-run flag for generation paths

Summary
Add a --dry-run flag to show what CommitLM would generate without writing files or modifying commit messages.

Scope

• commitlm generate --dry-run prints the synthesized commit message and/or doc path + preview.
• Respect in hooks via env var COMMITLM_DRY_RUN=1 (no file writes).

Tasks

• Wire flag at CLI and pass through to generators.
• Ensure no writes and exit code 0.
• Add examples to README.

Acceptance criteria

• Running with --dry-run never creates/changes files.
• Output includes which provider/model would be used.

Test plan
Stage a change; run commitlm generate --dry-run and verify only stdout changes.

[msaadsbr]

Hey, I would like to contribute.

[LeeSinLiang]

Hey, I would like to contribute.

Lovely. Fork the repo, and submit your PR when it's ready. The contribution guideline is here: https://github.com/LeeSinLiang/CommitLM/blob/main/CONTRIBUTING.md