feat: add --message-file for commit-msg hook support

[agherzan]

Enables validation of commit messages before the git object exists, allowing gitlance to work as a commit-msg hook. Comment lines are ignored, matching how a commit message is interpreted.

The new mode/argument is mutually exclusive with the ref-based options (--base, --head, --repo, --skip-merge-commits), which are only meaningful when validating existing commits.

[Copilot]

Pull request overview

This PR adds a new --message-file mode to validate a single commit message directly from a file (e.g., .git/COMMIT_EDITMSG), enabling gitlance to run as a commit-msg hook before a commit object exists.

Changes:

• Add --message-file CLI option and code path to validate a message read from disk, mutually exclusive with ref/range validation options.
• Add commit-msg git hook script and wire it into task install-githooks.
• Extend integration/unit tests to cover --message-file behavior and update CLI test harness.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/main.rs	Adds --message-file mode and ref-based validation branching / updated exit behavior.
src/git.rs	Introduces Commit::from_message() and comment-stripping for message-file mode, plus unit tests.
tests/integration_tests.rs	Updates integration harness to support message-file mode and adds message-file integration tests.
githooks/commit-msg	New hook to run gitlance --message-file during git commit.
Taskfile.yml	Installs the new commit-msg hook alongside pre-push.

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[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated no new comments.

[Copilot]

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Comments suppressed due to low confidence (2)

tests/integration_tests.rs:379

• file_path.to_str().unwrap() can panic without context if the path is not valid UTF-8. Prefer expect(...) (as used elsewhere in this file) to make failures actionable.

            !run_check(
                Some("wip-fixup"),
                None,
                None,
                None,
                Some(file_path.to_str().unwrap())
            ),

tests/integration_tests.rs:412

• file_path.to_str().unwrap() can panic without context if the path is not valid UTF-8. Prefer expect(...) for a clearer panic message if conversion fails.

            !run_check(
                Some("all"),
                None,
                None,
                None,
                Some(file_path.to_str().unwrap())
            ),