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>

cmd/bd/merge.go

@@ -26,7 +26,7 @@ markers for unresolvable conflicts.
 
 Designed to work as a git merge driver. Configure with:
 
-  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