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_test.go

@@ -627,7 +627,7 @@ func TestInitMergeDriverAutoConfiguration(t *testing.T) {
 		}
 
 		// Pre-configure merge driver manually
-		if err := runCommandInDir(tmpDir, "git", "config", "merge.beads.driver", "bd merge %A %O %L %R"); err != nil {
+		if err := runCommandInDir(tmpDir, "git", "config", "merge.beads.driver", "bd merge %A %O %A %B"); err != nil {
 			t.Fatalf("Failed to set git config: %v", err)
 		}
 
@@ -776,7 +776,7 @@ func TestInitMergeDriverAutoConfiguration(t *testing.T) {
 			t.Fatalf("Failed to get merge.beads.driver: %v", err)
 		}
 		driver = strings.TrimSpace(driver)
-		expected := "bd merge %A %O %L %R"
+		expected := "bd merge %A %O %A %B"
 		if driver != expected {
 			t.Errorf("Expected merge.beads.driver to be %q, got %q", expected, driver)
 		}