Merge base branch into PR before version bump

[Copilot]

The action now merges the default branch into the PR branch before bumping version files, ensuring the PR is up-to-date before making changes.

Changes

• New Merge utility (lib/utils/merge.rb): Uses Octokit's merge API to merge base into head branch

  • Returns success/failure with message
  • Handles conflicts, already-up-to-date, and error scenarios

• Updated Bump#bump_everything: Refactored to call merge first

  • Aborts version bump if merge fails
  • Recalculates version data post-merge to account for potential file changes
  • Prepends merge status to bump message

• Test coverage: Added merge_spec.rb and updated existing specs to mock merge calls

Behavior

When /dobby version <level> is invoked:

1. Merge default branch → PR branch
2. If merge succeeds: bump versions
3. If merge fails: return error, skip bump

Example output for merge conflict:

### :boom: Merge failed :boom:

Failed to merge main into feature-branch: merge conflict detected

Example output for successful flow:

### Merge complete

Successfully merged main into feature-branch

### Bump version from 1.0.0 to 1.1.0
| File Name | Message |
|-----------|---------|
| lib/version.rb | Bump minor version from 1.0.0 to 1.1.0 |

Original prompt

Can you extend this action so the first thing it does is updates the PR from the default branch using merge strategy and then if that succeeds it then bumps the version in the relevant files.

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