fix: Auto-repair stale merge driver configs with invalid placeholders

Old bd versions (<0.24.0) installed merge driver with invalid %L/%R
placeholders. Git only supports %O (base), %A (current), %B (other).

Changes:
- mergeDriverInstalled() now detects %L/%R and returns false to trigger repair
- bd init automatically fixes stale configs during initialization
- bd doctor --fix also repairs stale configs
- Added comprehensive test coverage for auto-repair

Fixes: bd-3sz0
Epic: bd-tbz3 (all sub-issues now complete)

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

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

cmd/bd/init_test.go

@@ -791,6 +791,63 @@ func TestInitMergeDriverAutoConfiguration(t *testing.T) {
 			t.Errorf("Expected merge.beads.name to contain 'bd', got %q", name)
 		}
 	})
+
+	t.Run("auto-repair stale merge driver with invalid placeholders", func(t *testing.T) {
+		// Reset global state
+		origDBPath := dbPath
+		defer func() { dbPath = origDBPath }()
+		dbPath = ""
+
+		tmpDir := t.TempDir()
+		originalWd, err := os.Getwd()
+		if err != nil {
+			t.Fatalf("Failed to get working directory: %v", err)
+		}
+		defer os.Chdir(originalWd)
+
+		if err := os.Chdir(tmpDir); err != nil {
+			t.Fatalf("Failed to change to temp directory: %v", err)
+		}
+
+		// Initialize git repo
+		if err := runCommandInDir(tmpDir, "git", "init"); err != nil {
+			t.Fatalf("Failed to init git: %v", err)
+		}
+
+		// Configure stale merge driver with old invalid placeholders (%L/%R)
+		// This simulates a user who initialized with bd version <0.24.0
+		if err := runCommandInDir(tmpDir, "git", "config", "merge.beads.driver", "bd merge %L %R"); err != nil {
+			t.Fatalf("Failed to set stale git config: %v", err)
+		}
+
+		// Create .gitattributes with merge driver
+		gitattrsPath := filepath.Join(tmpDir, ".gitattributes")
+		if err := os.WriteFile(gitattrsPath, []byte(".beads/beads.jsonl merge=beads\n"), 0644); err != nil {
+			t.Fatalf("Failed to create .gitattributes: %v", err)
+		}
+
+		// Run bd init - should detect stale config and repair it
+		rootCmd.SetArgs([]string{"init", "--prefix", "test", "--quiet"})
+		if err := rootCmd.Execute(); err != nil {
+			t.Fatalf("Init failed: %v", err)
+		}
+
+		// Verify merge driver was updated to correct placeholders
+		driver, err := runCommandInDirWithOutput(tmpDir, "git", "config", "merge.beads.driver")
+		if err != nil {
+			t.Fatalf("Failed to get merge.beads.driver: %v", err)
+		}
+		driver = strings.TrimSpace(driver)
+		expected := "bd merge %A %O %A %B"
+		if driver != expected {
+			t.Errorf("Expected merge driver to be repaired to %q, got %q", expected, driver)
+		}
+
+		// Verify it no longer contains invalid placeholders
+		if strings.Contains(driver, "%L") || strings.Contains(driver, "%R") {
+			t.Errorf("Merge driver should not contain invalid %%L or %%R placeholders, got %q", driver)
+		}
+	})
 }
 
 // TestReadFirstIssueFromJSONL_ValidFile verifies reading first issue from valid JSONL