[FEATURE] Add `--dry-run` flag to preview generated commit message without staging or committing anything

[Sweksha-Kakkar]

✨ Enhancement: --dry-run flag for safe message preview

Summary

Gitbun currently has two modes: interactive (shows preview, asks for confirmation) and --auto (commits immediately). There is no way to only see what message would be generated without touching the Git index or creating a commit.

This is a common use case for:

• Developers testing a custom .gitbunrc config or customPrompt
• CI pipelines that want to validate commit message quality before committing
• Contributors checking gitbun's fallback output against their diff without side effects

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Proposed Behaviour

A new flag --dry-run (alias: -n) should:

1. Run the full gitbun pipeline (AI call or fallback, scope/type detection)
2. Print the generated commit message to stdout
3. Exit without staging any files or running git commit

Example output:

$ gitbun --dry-run

🔍 Dry run mode — no files will be staged or committed.

Generated commit message:
─────────────────────────────────────
feat(analyzer): add support for multi-file scope detection
─────────────────────────────────────

AI model used : deepseek-coder:6.7b (Ollama)
Confidence    : high
Files analyzed: src/analyzer/scopeDetector.ts, src/analyzer/summarizer.ts

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Why This Matters

Currently, the only way to test gitbun's output quality is to actually run it interactively or inspect source code. A --dry-run flag makes gitbun composable in shell scripts and safe to evaluate without risk of accidental commits — especially important in team onboarding or CI validation steps.

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Implementation Notes

• Should work alongside --no-ai to test fallback output: gitbun --dry-run --no-ai
• Should work with --model to compare model outputs: gitbun --dry-run --model llama3
• Exit code 0 on success (message generated), 1 on failure (AI error, no staged files)
• Output should go to stdout so it can be piped: gitbun --dry-run | grep "feat"

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Environment

• All platforms (Node.js 18+)
• gitbun v1.13.0 (main branch)

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Expected Behaviour

After this flag is implemented:

• gitbun --dry-run prints the generated commit message and exits with code 0 — no git commit is run, no files are staged
• gitbun --dry-run --no-ai tests the rule-based fallback output specifically
• gitbun --dry-run --model llama3 uses the specified model for generation without committing
• The flag is documented in the README usage table alongside existing flags
• gitbun --help includes --dry-run / -n with description: "Preview generated commit message without committing"

[Sweksha-Kakkar]

@nirvik34
Please assign it to me I will rasie the PR soon I am ready to start my work.

[Aasritha-sure]

Hi @nirvik34 ,

I would like to work on this feature under GSSoC'26.

I understand that Gitbun currently lacks a safe preview mode for generated commit messages. My approach would be to introduce a --dry-run (-n) flag that executes the complete message-generation pipeline (AI or fallback), displays the generated commit message and metadata to stdout, and exits without staging files or creating a commit.

I will ensure that:

• --dry-run works with existing flags such as --no-ai and --model.
• No git staging or commit operations are triggered in dry-run mode.
• Proper exit codes are returned for success and failure scenarios.
• Output remains pipe-friendly for CI/CD workflows.
• The new flag is added to --help output and documented in the README usage section.

Additionally, I will test interactive, AI, fallback, and model-specific flows to ensure the feature behaves consistently across platforms.

Kindly assign this issue to me. Thank you!