Add git history discovery miner (#132)

[Dimwiddle]

Description

Add a language-agnostic GitHistoryMiner that parses recent git log output into typed GIT_COMMIT discovery items, filters noisy conventional commit prefixes, enforces source-file presence, and maps non-repo/missing-git failures to NOT_INSTALLED.

Also wires the miner into default discovery registration, adds focused unit coverage for parsing/filtering/error behavior, and updates the discovery feature spec scenarios/acceptance criteria for git mining.

Type of Change

• New feature
• Tests
• Documentation update

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Testing

• make lint-fix
• make lint
• make test
• make pre-commit

Related Issues

Closes #132