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/init.go

@@ -790,7 +790,7 @@ func mergeDriverInstalled() bool {
 // installMergeDriver configures git to use bd merge for JSONL files
 func installMergeDriver() error {
 	// Configure git merge driver
-	cmd := exec.Command("git", "config", "merge.beads.driver", "bd merge %A %O %L %R")
+	cmd := exec.Command("git", "config", "merge.beads.driver", "bd merge %A %O %A %B")
 	if output, err := cmd.CombinedOutput(); err != nil {
 		return fmt.Errorf("failed to configure git merge driver: %w\n%s", err, output)
 	}