[feat] Custom triggers: trigger agents from sources beyond Composio

[mmabrouk]

What

Triggers today are limited to Composio-provided events. We want custom triggers so an agent can be started from sources beyond the Composio catalog, including webhooks and AI-configured or user-defined events.

Scope

• Define a trigger from a non-Composio source (for example a direct webhook).
• Support AI-configured and user-defined event definitions.
• Reuse the existing trigger-to-agent delivery path.

Related earlier design work: #5191 (AI-configured triggers).

Tracking issue for the public product roadmap.

[linear-code]

AGE-3999