Move schemas inside trigger

Move the top-level schema to only allow 3 options and move everything under trigger for the `*.agents.md`.

Something like this -
```md
---
name: Structured Reporter
description: Agent that declares input/response schemas, an example, and arbitrary metadata.
trigger:
type: http_trigger
args:
route: "structured-report"
methods: ["POST"]
input_schema:
type: object
required:
- subscription_id
- report_type
properties:
subscription_id:
type: string
description: Azure subscription identifier.
report_type:
type: string
enum:
- cost
- security
- inventory
response_schema:
type: object
required:
- status
- summary
properties:
status:
type: string
enum:
- ok
- error
summary:
type: string
findings:
type: array
items:
type: object
properties:
severity:
type: string
message:
type: string
response_example: |
{
"status": "ok",
"summary": "All resources nominal.",
"findings": []
}
metadata:
owner: platform-team
tags:
- reporting
- azure
cost_center: 4242
enabled: true
---

You are a reporting agent. Always return JSON matching the response schema.
```

This ensures everything stays cleanly isolated under the `triggers` top-level argument.

Start by locating the parser and validation logic for *.agents.md files, then find any tests covering top-level fields, trigger configuration, schemas, examples, or metadata. Done means the format accepts only the intended top-level options and keeps the schema-related configuration isolated under trigger, with tests covering the sample structure.

Written by the indexing model from the issue text.