Simplify seed data configuration by removing command line flags and renaming environment variable (#175)

This PR simplifies the seed data import configuration based on
maintainer feedback by removing command line flags and using a clearer
environment variable name.

## Key Changes

### Configuration Simplification
- **Renamed**: `MCP_REGISTRY_SEED_FILE_PATH` → `MCP_REGISTRY_SEED_FROM`
- **Removed**: `--seed-file-path` command line flag to eliminate
configuration complexity

### Improved User Experience
Users now configure seed data import using a single, clear environment
variable:

```bash
# Before (multiple ways to configure)
MCP_REGISTRY_SEED_FILE_PATH=data/seed.json ./registry
./registry --seed-file-path=data/seed.json

# After (simplified single method)
MCP_REGISTRY_SEED_FROM=data/seed.json ./registry
```

### Registry Composability
The composability workflow is now simplified while maintaining all
functionality:

```bash
# Start first registry with seed data
MCP_REGISTRY_SEED_FROM=data/seed.json ./registry

# Start second registry importing from first via pagination
MCP_REGISTRY_SEED_FROM=http://localhost:8080 ./registry
```

## Technical Implementation
- Maintains full backward compatibility for import functionality
- All existing seed data sources continue to work (local files, HTTP
URLs, registry URLs)
- Automatic pagination through `/v0/servers` endpoint remains unchanged
- Configurable delays between requests preserved

## Benefits
- **Reduced complexity**: Single configuration method instead of
multiple
- **Clearer naming**: `SEED_FROM` better describes the source of seed
data
- **Easier maintenance**: Fewer configuration paths to test and document
- **Consistent patterns**: Aligns with other environment-variable-only
configuration

All existing functionality is preserved while providing a cleaner, more
maintainable configuration interface.

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Avinash Sridhar <sridharavinash@users.noreply.github.com>
Co-authored-by: Toby Padilla <toby@toby.sh>
Co-authored-by: toby <83556+toby@users.noreply.github.com>

cmd/registry/main.go

@@ -79,14 +79,14 @@ func main() {
 		return
 	}
 
-	// Import seed data if requested (works for both memory and MongoDB)
-	if cfg.SeedImport {
-		log.Println("Importing data...")
+	// Import seed data if seed source is provided (works for both memory and MongoDB)
+	if cfg.SeedFrom != "" {
+		log.Printf("Importing data from %s...", cfg.SeedFrom)
 		ctx, cancel := context.WithTimeout(context.Background(), 5*time.Minute)
 		defer cancel()
 
-		if err := db.ImportSeed(ctx, cfg.SeedFilePath); err != nil {
-			log.Printf("Failed to import seed file: %v", err)
+		if err := db.ImportSeed(ctx, cfg.SeedFrom); err != nil {
+			log.Printf("Failed to import seed data: %v", err)
 		} else {
 			log.Println("Data import completed successfully")
 		}