History backfill command

[LeeSinLiang]

History backfill command

Summary
Add commitlm backfill to generate documentation for existing commits (e.g., to seed a new project knowledge base).

Motivation
Creates historical docs for onboarding and auditing.

CLI proposal

commitlm backfill [--since <rev> | --last N] [--until <rev>] [--reverse] [--dry-run] [--provider ... --model ...]

Scope

• Writes to docs/history/<short-hash>_<slug>.md (same content structure as post-commit docs).
• Rate-limit aware (configurable delay).
• Respects Include/Exclude patterns (see related issue).

Tasks

• Implement using Git plumbing (git rev-list) or GitPython.
• Respect include/exclude patterns.
• Add --dry-run to print planned work only.
• Docs + examples.

Acceptance criteria

• Backfills N commits deterministically with or without --reverse.
• Idempotent: re-running doesn’t duplicate.
• Works with local and cloud providers.

Test plan
Create a repo with 5 commits; run commitlm backfill --last 3; verify 3 files created; re-run is a no-op.