Add keyword filtering to `agentseek create --list-templates`

Summary

Add a keyword filter to template listing so users can quickly find relevant templates as the catalogue grows.

Requirements

• Support agentseek create --list-templates --filter <keyword>.
• Match the keyword against template specs and descriptions.
• Filtering should work for all templates and for a selected type, for example agentseek create langchain --list-templates --filter langgraph.
• Matching should be case-insensitive.
• Keep the current unfiltered output unchanged.

Acceptance Criteria

• Filtered output only includes matching templates.
• Non-matching filters print a clear empty-result message.
• Unit tests cover all-template filtering, type-specific filtering, and no-match output.