PSMDB-1965: add instructions for merging workflow

.github/MERGAI.md

@@ -0,0 +1,92 @@
+# Mergai Workflow
+
+The `mergai.yml` workflow automates the process of merging upstream MongoDB commits into Percona Server for MongoDB, including AI-assisted conflict resolution and AI-assisted summary of merged commits.
+
+## Quick Start
+
+1. **Check available commits**: Visit the [Fork Status page](https://percona.github.io/percona-server-mongodb/)
+2. **Trigger merge**: Go to Actions → "mergai bot" → "Run workflow", or use CLI:
+
+   ```bash
+   gh workflow run .github/workflows/mergai.yml -f merge-pick=<SHA>
+   ```
+
+3. **Review the PR**: The workflow creates one of two PR types:
+   - **Merge PR** - Clean merge without conflicts, ready for review
+   - **Conflict Solution PR** - Contains conflict markers with AI-resolved solutions visible in the diff for easy review
+4. **Build and test**: Check out the branch locally, build, and run tests
+5. **Merge**: Post `/fast-forward` comment to merge without double commits
+
+## Workflow Jobs
+
+| Job               | Trigger                          | Purpose                                              |
+| ----------------- | -------------------------------- | ---------------------------------------------------- |
+| `trigger-merge`   | Manual dispatch                  | Performs merge, handles conflicts with AI resolution |
+| `solution-merged` | PR merged to `mergai/*/conflict` | Finalizes after solution PR is merged                |
+
+## Understanding PRs
+
+### Merge PR (no conflicts)
+
+- Direct PR to main branch
+- Review "Auto-merged files" section
+
+### Conflict Resolution PR
+
+- PR from `solution` branch into `conflict` branch
+- Check "Conflict Context" and "Solutions" sections for AI-resolved conflicts
+- Unresolved files will have conflict markers visible in review
+
+## Making Fixes
+
+If tests fail or changes are needed:
+
+### Option 1 - Simple (commits not tracked by mergai)
+
+Add commits directly to the branch. This works but commits won't be marked as solutions for future reuse.
+
+### Option 2 - Tracked (recommended)
+
+```bash
+# Add your commits, then:
+mergai commit sync       # Add mergai notes to commits
+mergai branch push main  # Push branch
+mergai notes push        # Push notes
+mergai pr update main    # Update PR description
+```
+
+**Optional**: Use `mergai commit squash` to squash all commits into the merge commit.
+
+## Merging the PR
+
+Ensure the PR branch is rebased onto (and fast-forwardable to) the base branch, then post a comment with `/fast-forward` to merge without creating extra merge commits.
+
+## Canceling a Merge
+
+To cancel an in-progress merge:
+
+1. Close the PR
+2. Delete the associated branch(es)
+
+## Local Setup (Optional)
+
+For local work with mergai:
+
+```bash
+mergai config           # One-time: configure git for note markers and diff3
+mergai notes update     # Sync notes from remote
+mergai context init     # Create local context from commit notes
+# Now other mergai commands work
+```
+
+If you need to rebase an existing merge:
+
+```bash
+mergai context init
+mergai rebase
+```
+
+## Resources
+
+- [Mergai Documentation](https://github.com/Percona-Lab/mergai#readme)
+- [Fork Status Dashboard](https://percona.github.io/percona-server-mongodb/)