fix: backtrack workflow leaves concrete branch rules behind after automated merges (#193)

The backtrack workflow locked concrete branches (e.g., `preview/1.0`,
`develop/1.0`) during automated merges but never properly cleaned them
up — the "restore" steps called the `lock` action with `lock-branch:
false`, which issues a `PUT` that modifies the rule but leaves the full
protection ruleset in place. Branches with no prior protection ended up
permanently protected after each run.

## Changes

- **`backtrack.yml`** — Replace all three "Restore protection" steps
with the `unlock` action (`DELETE /branches/{branch}/protection`)
instead of `lock` with `lock-branch: false` (`PUT
/branches/{branch}/protection`). Concrete per-branch rules are fully
removed after the merge; the manually-maintained glob pattern rules
(`develop/**`, `preview/**`, `release/**`) remain as the base
protection.

```yaml
# Before — PUT: flips lock_branch off but leaves the full ruleset behind
- uses: './.github/actions/github/branch-protection/lock'
  with:
    branch: ${{ steps.targets.outputs.develop-branch }}
    token: ${{ secrets.GH_ADMIN_TOKEN }}
    lock-branch: 'false'

# After — DELETE: removes the temporary concrete rule entirely
- uses: './.github/actions/github/branch-protection/unlock'
  with:
    branch: ${{ steps.targets.outputs.develop-branch }}
    token: ${{ secrets.GH_ADMIN_TOKEN }}
    fail-on-error: 'true'
```

Author: Copilot

.github/workflows/backtrack.yml

@@ -127,27 +127,27 @@ jobs:
 
       - name: 'Restore protection: base branch'
         if: ${{ always() && steps.lock-base.outcome == 'success' }}
-        uses: './.github/actions/github/branch-protection/lock'
+        uses: './.github/actions/github/branch-protection/unlock'
         with:
           branch: ${{ github.base_ref }}
           token: ${{ secrets.GH_ADMIN_TOKEN }}
-          lock-branch: 'false'
+          fail-on-error: 'true'
 
       - name: 'Restore protection: preview branch'
         if: ${{ always() && steps.lock-preview.outcome == 'success' }}
-        uses: './.github/actions/github/branch-protection/lock'
+        uses: './.github/actions/github/branch-protection/unlock'
         with:
           branch: ${{ steps.targets.outputs.preview-branch }}
           token: ${{ secrets.GH_ADMIN_TOKEN }}
-          lock-branch: 'false'
+          fail-on-error: 'true'
 
       - name: 'Restore protection: develop branch'
         if: ${{ always() && steps.lock-develop.outcome == 'success' }}
-        uses: './.github/actions/github/branch-protection/lock'
+        uses: './.github/actions/github/branch-protection/unlock'
         with:
           branch: ${{ steps.targets.outputs.develop-branch }}
           token: ${{ secrets.GH_ADMIN_TOKEN }}
-          lock-branch: 'false'
+          fail-on-error: 'true'
 
       - name: 'Write backtrack summary'
         if: ${{ always() }}