Add /gh-notif-triage skill for clearing notification inbox noise

[ikuwow]

Closes #186.

Why

The notification inbox accumulates threads that rarely deserve attention (CI results, subscription updates, close / merge notices). Clearing them by hand is toil. The skill encodes the rule-based classification the issue settled on, so the work fits into a single /gh-notif-triage invocation.

What

• claude/skills/gh-notif-triage/SKILL.md follows the design from Add /gh-notif-triage Skill: triage GitHub notifications and mark noise as done #186 verbatim. Argument handling: bare invocation lists unread, all includes already-read, auto skips the approval step.
• The rule table for reason → done | keep lives at the top of the skill so the user can edit it inline as preferences change.
• claude/settings.json adds Skill(gh-notif-triage) and Skill(gh-notif-triage *) to allow.

Notes

• First cut intentionally omits the write auto-approve hook from Add /gh-notif-triage Skill: triage GitHub notifications and mark noise as done #186's "Future work" section. Each DELETE still surfaces an ask prompt because of the Bash(gh api * -X *) rule in settings.json; that is acceptable for the initial workflow and matches the issue's call.
• No persistent log of done threads. Recoverability is not a goal: the kept categories (review_requested / mention / assign) are reachable from other GitHub views.

Verification

• gh api '/notifications?all=false&per_page=1' --jq '. | length' returns a valid count — the endpoint the skill depends on is reachable from this user.
• jq . claude/settings.json validates.
• /gh-notif-triage (live invocation) lists candidates and waits for approval before any DELETE — deferred to first real use to avoid surfacing a fresh permission prompt mid-batch.
• /gh-notif-triage all widens the fetch to read+unread.
• /gh-notif-triage auto skips approval.
• Deleting a known-safe ci_activity notification confirms it leaves the inbox (gh api '/notifications?all=false') — defer until the first organic triage so a real ci_activity exists to clear.