gh-monitor: follow-ups after v1 (ci + inbox)

[aaronsb]

v1 of the gh-monitor skill shipped in #149 (ci + inbox modes). Three follow-ups were deliberately deferred:

• comments mode — watch one PR/issue thread for new replies (repos/{o}/{r}/issues/{n}/comments?since=). Lowest value of the three; the Monitor tool docs ship this exact pattern.
• Retire tools/attend/examples/gh-notifications.sh — now that inbox mode covers notifications via Monitor, the example sensor is the same antipattern the GitHub sensors were retired for (ADR-137). Decide whether to delete it or keep it purely as a sensor-contract teaching example.
• Proactive nudge-way — a thin way that fires on git push / PR creation and suggests /gh-monitor ci. Per the Knowledge Way: the skill gives Claude something to run; a way can nudge toward it.

Possible v2 efficiency upgrade also noted in inbox-watch.sh: swap the since= marker for If-None-Match + stored ETag → 304 on idle polls (verified supported; pure rate-limit efficiency, not correctness).

Context: ADR-137 (boundedness) established external/session-specific watching is a Monitor concern, not an attend sensor. #25 / #44 closed as wrong-shape; #146 retired the gh-pr-checks sensor; #149 shipped the correctly-shaped replacement.