feat: inject platform base_url into agent system prompt and webhook URLs

[nap-liu]

Summary

• Agent system prompt now includes platform URL patterns (webhook, public page, file download, gateway)
• list_triggers returns full webhook URLs instead of relative paths
• Uses existing platform_service.get_public_base_url()

Test plan

• E2E: asked Morty for platform URL → correctly returned all 5 URL patterns

[nap-liu]

This PR is small and independent (2 files, +33/-3) and merges cleanly against current upstream/main. It injects the platform base_url into agent system prompts and webhook URLs so agents can generate correct public links regardless of how the request reached the backend.

Would appreciate a dedicated review here rather than through the PRG-02 bundle (#366).