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/doctor/fix/merge_driver.go

@@ -0,0 +1,25 @@
+package fix
+
+import (
+	"fmt"
+	"os/exec"
+)
+
+// MergeDriver fixes the git merge driver configuration to use correct placeholders.
+// Git only supports %O (base), %A (current), %B (other) - not %L/%R.
+func MergeDriver(path string) error {
+	if err := validateBeadsWorkspace(path); err != nil {
+		return err
+	}
+
+	// Update git config to use correct placeholders
+	// #nosec G204 -- path is validated by validateBeadsWorkspace
+	cmd := exec.Command("git", "config", "merge.beads.driver", "bd merge %A %O %A %B")
+	cmd.Dir = path
+	output, err := cmd.CombinedOutput()
+	if err != nil {
+		return fmt.Errorf("failed to update git merge driver config: %w\nOutput: %s", err, output)
+	}
+
+	return nil
+}