claude-plugins: add forge-triage skill

claude-plugins/plugins/base/skills/forge-triage/SKILL.md

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+---
+name: forge-triage
+description: Verify triage coverage on a forge repo using the existing lint verbs instead of manually scanning issues/PRs - unassigned items, missing labels by scope, PRs with no reviewer, and stale branches each have a dedicated check. Use this whenever you want to confirm nothing has fallen through the cracks (new activity landed, a periodic sweep, before reporting "everything's triaged").
+---
+
+# Forge Triage
+
+Checking triage coverage doesn't need a bespoke script or a manual scan
+of the tracker - the forge CLI's `lint` verbs already answer each
+dimension directly.
+
+## The checks
+
+- **Unassigned items** - open issues or PRs with no assignee:
+  `hive-forge lint unassigned`
+- **Missing labels by scope** - items missing any label in a given
+  scope (e.g. every issue should carry a `type/*` label, or an `area/*`
+  one): `hive-forge lint unlabeled --scope <scope>`. The scope name is
+  whatever your repo's own label taxonomy uses - there's nothing
+  hardcoded, so check `repo-labels` first if you don't already know the
+  scopes in use.
+- **PRs with no reviewer engagement** - `hive-forge lint no-reviewer
+  --reviewer <name>` flags PRs where the named reviewer hasn't left a
+  formal review or mention. The reviewer name is per-repo/per-team, not
+  a fixed value.
+- **Assignment load** - `hive-forge lint assignments` groups open
+  issues + PRs by assignee, useful for spotting an overloaded or
+  neglected owner.
+- **Stale branches** - `hive-forge lint stale-branches --days <n>`
+  finds remote branches with no recent commits (skips branches that are
+  heads of open PRs), useful for spotting abandoned work.
+
+## Using them as a sweep
+
+Run the checks relevant to what you're verifying rather than assuming
+one covers everything - "is triage caught up" usually means unassigned
++ unlabeled at minimum, with no-reviewer added if reviews matter for
+your workflow. Each verb supports `--json` if you want to fold the
+results into something else instead of reading the human-readable output.
+
+This is reactive/on-demand coverage checking, not a scheduled rollup or
+historical trend - if you need to track compliance *over time* (was
+triage worse last week than this week), that's a different, bigger
+ask than running these checks - don't build that unprompted just
+because the checks exist.