Fix bd-afd: Auto-fix metadata.json jsonl_export mismatch

## Summary

When metadata.json gets deleted (git clean, merge conflict, rebase), the
version tracking code auto-recreates it using DefaultConfig() which hardcoded
jsonl_export to 'issues.jsonl'. But many repos (including beads itself) use
'beads.jsonl', causing a mismatch between config and actual JSONL file.

## Changes

1. **bd doctor --fix auto-detection** (cmd/bd/doctor/fix/database_config.go)
   - New DatabaseConfig() fix function that auto-detects actual JSONL file
   - Prefers beads.jsonl over issues.jsonl (canonical name)
   - Skips backup files and merge artifacts
   - Wired into doctor.go applyFixes()

2. **Version tracking auto-detection** (cmd/bd/version_tracking.go)
   - trackBdVersion() now scans for existing JSONL files before defaulting
   - Prevents mismatches when metadata.json gets recreated
   - Added findActualJSONLFile() helper function

3. **Canonical default name** (internal/configfile/configfile.go)
   - DefaultConfig() changed from issues.jsonl to beads.jsonl
   - Aligns with canonical naming convention

4. **FindJSONLPath preference** (internal/beads/beads.go)
   - Now prefers beads.jsonl over issues.jsonl when scanning
   - Default changed from issues.jsonl to beads.jsonl

5. **Test coverage**
   - Added comprehensive tests for DatabaseConfig fix
   - Updated configfile tests for new default
   - Verified backup file skipping logic

## Testing

- All existing tests pass
- New tests verify auto-fix behavior
- Integration tested with simulated mismatches

Closes: bd-afd

internal/configfile/configfile.go

@@ -18,7 +18,7 @@ type Config struct {
 func DefaultConfig() *Config {
 	return &Config{
 		Database:    "beads.db",
-		JSONLExport: "issues.jsonl",
+		JSONLExport: "beads.jsonl", // Default to canonical name (was issues.jsonl)
 	}
 }
 

internal/configfile/configfile_test.go

@@ -13,8 +13,9 @@ func TestDefaultConfig(t *testing.T) {
 		t.Errorf("Database = %q, want beads.db", cfg.Database)
 	}
 
-	if cfg.JSONLExport != "issues.jsonl" {
-		t.Errorf("JSONLExport = %q, want issues.jsonl", cfg.JSONLExport)
+	// bd-afd: Default changed from issues.jsonl to beads.jsonl (canonical name)
+	if cfg.JSONLExport != "beads.jsonl" {
+		t.Errorf("JSONLExport = %q, want beads.jsonl", cfg.JSONLExport)
 	}
 }