feat(update): rebase local commits instead of discarding on update

[GratefulDave]

Problem

When hermes update is run and the local branch has commits ahead of origin/main (e.g. cherry-picked patches, local plugin hooks, config customizations), the update flow does git reset --hard origin/main — silently destroying all local commits.

This is especially painful for users who:

• Maintain local patches on top of Hermes (plugins, hooks, provider fixes)
• Cherry-pick features from PRs before they merge
• Have any local customization committed to their main branch

Fix

Detect local commits before pulling. When fast-forward fails:

1. Local commits exist → git rebase origin/main to replay local commits on top of the new upstream
   • If rebase hits conflicts → abort cleanly, report which commits are affected, and give manual resolution instructions
   • Never silently discard or auto-resolve
2. No local commits (pure divergence from upstream force-push) → git reset --hard origin/main (same as before)
3. ff-only succeeds → show notification if local commits were preserved alongside upstream updates

Before / After

Before: hermes update with 1 local commit → commit silently destroyed, user confused

After: hermes update with 1 local commit → rebase preserves it, user sees:

→ Found 12 new commit(s)
→ Pulling updates...
  ⚠ Fast-forward not possible (have 1 local commit(s)), rebasing...
  → Updating Python dependencies...

Or if conflicts:

✗ Rebase failed due to merge conflicts.
  Your 1 local commit(s) were NOT lost.
  Stashed changes are preserved (ref: abc123).

  To resolve manually:
    git rebase origin/main
    # fix conflicts, then: git rebase --continue
    git stash apply abc123

Testing

• Local commits + upstream update: rebase succeeds, commits preserved
• Local commits + conflicting upstream update: rebase aborts, clear instructions
• No local commits + diverged: reset --hard (unchanged behavior)
• No local commits + ff succeeds: normal update (unchanged)

[GratefulDave]

Closing — needs a fresh rebase against current main. Will resubmit if still relevant.