Fix agent-start workflow trigger: use @agent mention, drop dead comment context refs

## Problem

The active workflow at `.github/workflows/agent-start.yml` only listens to `issues: [labeled]`, but its `if:` condition references `github.event.comment.*` and event names `issue_comment` / `pull_request_review_comment`. Since the `comment` object doesn't exist in the `issues` event payload, the Actions linter reports:

```
Context access might be invalid: comment
```

Those comment/PR-review branches are effectively dead code — they can never fire under the current `on:` triggers.

## Goal

Trigger the pipeline on an `@agent` mention (not the label), across:
- New/edited issues whose body contains `@agent`
- Issue comments containing `@agent`
- PR review comments containing `@agent`

## Fix

- Expand `on:` to include `issues: [opened, edited]`, `issue_comment: [created]`, and `pull_request_review_comment: [created]` so the referenced contexts actually exist.
- Keep the author-association / actor authorization gate.
- Remove the label dependency (the pipeline payload never consumed labels anyway).

The label is not required by the pipeline: the Step Functions payload is built purely from the issue GraphQL query and contains no labels. The `agent:start` Cedar policy only *forbids* the agent from setting the label (loop prevention), it does not gate invocation.

Start with .github/workflows/agent-start.yml and inspect its current on: triggers and if: condition. Run the GitHub Actions linter, then verify that opened or edited issues, issue comments, and pull request review comments containing @agent can pass the existing authorization gate without depending on labels.

Written by the indexing model from the issue text.