feat(git-harness): extract git and safety primitives

[bschellenberger2600]

Summary

Extract the first standalone git-harness library slice by copying git-fire primitives into this repository:

• add go.mod for module github.com/git-fire/git-harness (go 1.24.2)
• add go.sum for resolved module checksums
• add git/ package with repository scanning and git operation primitives
• add safety/ package with redaction and secret scanning helpers
• port corresponding tests for both packages

This intentionally does not integrate the library back into git-fire yet.

Why this change?

This kicks off the git-harness plan by isolating reusable git logic out of git-fire first, so we can harden the standalone module before later consumer import rewrites.

Bug Reproducibility (if this fixes a bug)

• Repro steps:
  1. N/A
  2. N/A
  3. N/A
• If reproducibility steps are not feasible, explain why:

No direct bug fix in this phase; this is extraction/scaffolding work.

Tests

• I added or updated tests for all code changes in this PR.
• For bug fixes, I added regression coverage.
• I ran go test -race -count=1 ./....
• I ran go vet ./....

Additional verification:

• go build ./...

Risk / Rollback Notes (optional)

Low runtime risk to consumers because this PR does not wire any external consumers yet. Rollback is straightforward by reverting this branch/PR.