feat: lint commit message from hash range

[JustBipin]

Adds --from-hash and --to-hash flags to lint all commit messages between two commits, inclusive.

Usage

cocox --from-hash <hash>              # from <hash> to HEAD
cocox --from-hash <hash> --to-hash <hash>   # explicit range

Feature

• --from-hash and --to-hash flags added to the CLI
• --to-hash defaults to HEAD when not provided
• --to-hash requires --from-hash and is mutually exclusive with message, --file, and --hash
• src/git_helpers.rs: added get_commit_messages_from_hash_range() which fetches all commit messages in the range and prepends the from commit separately, since A..B in git excludes A

Test infrastructure

• tests/common/git_repo.rs: introduces TestRepo, a temporary git repo with minimal config for use in integration tests
  • TestRepo::new() switches the process working directory into the temp repo
  • Drop restores the original working directory when TestRepo goes out of scope
  • A static Mutex is acquired on construction and released on drop, serializing all tests that use TestRepo to prevent race conditions on the process-global cwd
• serial_test = "3" added as a dev-dependency
• Unit tests in src/git_helpers.rs that depended on the project repo's own git history replaced with integration tests in tests/git_helpers.rs using TestRepo, so tests are self-contained and repo-state independent

Integration tests added

• tests/git_helpers.rs: full coverage of get_commit_message_from_hash and get_commit_messages_from_hash_range with known commit messages
• tests/cli.rs: success and failure cases for --hash (exact hash, invalid message, ignored message) and --from-hash/--to-hash (invalid commit at from, middle, and to positions; ignored commits in range; single-commit range)
• tests/cli.rs: extended mutual exclusion tests covering all missing argument combinations (message + --hash, message + --from-hash, --hash + --from-hash, --to-hash with non---from-hash inputs)

TestCoverage: 97%

closes: #10, #12