fix: Correct git merge driver placeholders from %L/%R to %A/%B

Git merge drivers only support three placeholders:
- %O (ancestor/base)
- %A (current version)
- %B (other branch's version)

The code was incorrectly using %L and %R, which don't exist in git,
causing them to be passed through literally and breaking JSONL merges.

Changes:
- Fixed merge driver config in init.go, merge.go, README.md, docs
- Added detection in bd doctor with clear error messages
- Added auto-fix in bd doctor --fix
- Added proactive warning in bd sync before git pull
- Added reactive error detection after merge failures
- Updated all tests to use correct placeholders

Now users get helpful guidance at every step:
1. bd doctor detects the issue
2. bd doctor --fix auto-corrects it
3. bd sync warns before pulling if misconfigured
4. Error messages suggest bd doctor --fix when merge fails

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

README.md

@@ -156,7 +156,7 @@ echo "BEFORE ANYTHING ELSE: run 'bd onboard' and follow the instructions" >> AGE
 
 **Git merge driver:** During `bd init`, beads configures git to use `bd merge` for intelligent JSONL merging. This prevents conflicts when multiple branches modify issues. Skip with `--skip-merge-driver` if needed. To configure manually later:
 ```bash
-git config merge.beads.driver "bd merge %A %O %L %R"
+git config merge.beads.driver "bd merge %A %O %A %B"
 git config merge.beads.name "bd JSONL merge driver"
 echo ".beads/beads.jsonl merge=beads" >> .gitattributes
 ```