* feat(skills): watchers skill — poll RSS / HTTP JSON / GitHub via cron no-agent
Ships three reusable polling scripts plus a shared watermark helper as an
optional skill. Users wire them into the existing cron (no_agent=True)
mode rather than learning a new subsystem.
Supersedes the closed PR #21497 (parallel watcher subsystem). Same value,
zero new core surface.
## What ships
- optional-skills/devops/watchers/SKILL.md: pattern + three example cron commands
- optional-skills/devops/watchers/scripts/_watermark.py: shared helper
(atomic state writes, bounded ID set, first-run baseline)
- optional-skills/devops/watchers/scripts/watch_rss.py: RSS 2.0 + Atom
- optional-skills/devops/watchers/scripts/watch_http_json.py: any JSON endpoint
with configurable id_field / items_path / headers
- optional-skills/devops/watchers/scripts/watch_github.py: issues / pulls /
releases / commits (uses GITHUB_TOKEN if present)
## Invariants enforced by the shared helper
- First run records baseline, emits nothing (never replays existing feed)
- Watermark file is <state_dir>/<name>.json, atomic replace on write
- Bounded to 500 IDs (configurable)
- Empty stdout when no new items — cron treats that as silent delivery
## Validation
- watch_rss.py against news.ycombinator.com/rss first run → empty stdout, watermark populated
- Removed one seen-id, second run → emitted exactly that item
- No DeprecationWarnings (ET element truth-value footgun dodged explicitly)
End-user pattern: 'hermes cron create my-feed --schedule "*/15 * * * *" --no-agent --script $HERMES_HOME/skills/devops/watchers/scripts/watch_rss.py --script-args "--name hn --url https://news.ycombinator.com/rss" --deliver telegram'
* docs(skills/watchers): tighten description to match peer optional skills
* docs(skills/watchers): align frontmatter + structure with peer optional skills
* docs(skills/watchers): gate to linux/macos (shell syntax in examples)
