Add issue-triage skill and board audit, for community inflow #4868

Part of the agentic software factory tracker #4858, Wave 3. Proposed as S7 in docs/agent-guidance-inception.md (PR #4861) §9, from #4859.

Depends on

After Part 6 (board-ops/issue-create). Also after Part 9 (#4869). Outside PRs are being turned off, so this pass closes each outside PR with a pointer to the issue flow, per the backlog plan in Part 9.

Scope

• Adapt the Inspector's issue-triage skill (§6 of the doc):
  • the two-pass sweep: board as Incoming, then human approval
  • the priority rubric, with the score posted as a comment
  • the board audit, with every check printing 0, including the [GHSA-…] draft-card carve-out if Part 10 (Add security-advisory skill; reconcile SECURITY.md with the advisory backlog #4870) has already landed (otherwise Part 10 adds it)
  • reword the rubric's severity axis for servers
  • raise the audit's --limit above this repo's total issue count
• Add triage classes for this repo's inflow, each with a canned response and a close/label action:
  • server submissions and "add my server" README / ADDITIONAL.md PRs, redirected to the Registry
  • new server implementations
  • duplicate fixes racing for the same bug
  • no-op or rename PRs
• Fold in the behavior of readme-pr-check.yml.
• At the time of writing: 245 open issues and 316 open PRs.

Acceptance criteria

• A triage pass runs over the current open backlog, and the audit prints all zeros afterwards.
• Each spam class has a documented response.
• The open outside-PR backlog is handled per Part 9's plan.
• The skill carries eval cases that pass the threshold.

Branch

Targets v2/main. Closes #N won't auto-close an issue on a non-default branch, so close this one by hand on merge and move the card to Done.