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.go

@@ -752,7 +752,7 @@ exit 0
 	return nil
 }
 
-// mergeDriverInstalled checks if bd merge driver is configured
+// mergeDriverInstalled checks if bd merge driver is configured correctly
 func mergeDriverInstalled() bool {
 	// Check git config for merge driver
 	cmd := exec.Command("git", "config", "merge.beads.driver")
@@ -761,6 +761,14 @@ func mergeDriverInstalled() bool {
 		return false
 	}
 
+	// Check if using old invalid placeholders (%L/%R from versions <0.24.0)
+	// Git only supports %O (base), %A (current), %B (other)
+	driverConfig := strings.TrimSpace(string(output))
+	if strings.Contains(driverConfig, "%L") || strings.Contains(driverConfig, "%R") {
+		// Stale config with invalid placeholders - needs repair
+		return false
+	}
+
 	// Check if .gitattributes has the merge driver configured
 	gitattributesPath := ".gitattributes"
 	content, err := os.ReadFile(gitattributesPath)