Feature: Add GitHub verifiers for the watch system (issue_updated, pr_status, etc.) #18

Summary

The create_watch tool accepts a check parameter for a registered plugin verifier, but the GitHub plugin currently doesn't expose any verifiers (e.g. github:issue_updated, github:issue_comments, github:issue_activity). Attempts to use GitHub-related verifier names all fail with a check_args validation error, suggesting the verifier isn't recognized at all.

Motivation

Watches are a core agent capability — they let the agent monitor external state changes and act on them. Without GitHub verifiers, agents can't:

• Watch an issue for triage (label changes, comments, assignees)
• Watch a PR for CI status changes
• Watch a repo for new releases or branches
• Trigger follow-up actions when GitHub state changes

This is a gap between the watch system's design and the GitHub plugin's capabilities.

Proposed Verifiers

Suggested verifier names and what they'd check:

Each would take check_args like {"repo": "owner/name", "number": 123} or {"repo": "owner/name"} depending on scope.

Acceptance Criteria

• At least github:issue_updated and github:pr_status are registered as usable verifiers for create_watch
• The verifiers correctly detect state changes on the specified issue/PR
• The watch fires run_prompt in-session when a change is detected
• Docs/tutorials mention how to use GitHub watches

Notes

• The GitHub plugin already has tools like github_get_issue, github_get_pr, github_ci_runs — the verifiers would wrap these with polling + change detection.
• This is a plugin-level feature, not a core agent change.