feat(workflows): add log-watcher community AW

[adamhenson]

Adds workflows/log-watcher.md - a community agentic workflow that fires after monitored agent workflow runs complete, downloads the agent-artifacts artifact written by gh-aw's firewall, scans run logs and token data for error patterns, retry loops, and anomalies, then posts a diagnostic health report on the associated PR or creates a diagnosis issue.

This workflow is referenced from the githubnext/agentics README as a community AW maintained in this repo.

What it detects

• Error, exception, and fatal signals in run logs
• Timeout and rate-limit signals (429, context deadline)
• Retry and tool-call loops (same tool called more than 5 times)
• Context window truncation warnings
• Token anomalies: high output ratio, low cache efficiency, total token spikes, unexpected model mixing

Three health levels

• Healthy: brief one-liner, no noise
• Degraded: warnings or flagged metrics present
• Failed: critical errors or non-success conclusion

Related to githubnext/agentics#327.