Add event filters for coding agent workflows

Why

A coding agent should run when someone asks for it, not on every comment. An event trigger can currently select an action but not who the event addresses or who caused it, so an agent workflow either runs too often or needs a shell check that cannot stop the run from being queued.

What changes

• mentioned, replied-to, and assigned-to select events that address an account.
• author.name and author.role limit who can trigger the workflow. author.role: maintain keeps contributors and readers out.
• Issue events gain assigned, and event payloads carry mentions, reply authors, and assignee changes.
• Mention parsing is shared and ignores code samples, email addresses, and URLs.
• author can be written as one account or a list as shorthand for author.name.

on:
  comment:
    types: [created]
    mentioned: [coding-agent]
    replied-to: [coding-agent]
    author:
      role: maintain

Checks

• vole workflow check
• vole workflow run verify

Related

Part of #138 (workflow format gaps). This does not close it.