Pause issue detection: keep plumbing, replace output with placeholder

[DarinShapiro]

Summary

Keep the Issues storage, API surface, and MCP tool wiring in place, but stop generating issues. Replace the dashboard card and API payload with a clearly-labeled placeholder until the rules are redesigned (see companion tracking issue).

Rationale (from discussion): the current issue rules largely restate state that's already visible elsewhere on the dashboard (partition count, phantom nodes, stale links). Worse, they bias AI consumers toward a specific diagnostic path that isn't always correct. Better to surface nothing than to surface confidently-wrong-shaped guidance.

Scope

Pipeline

• Disable the rule evaluators that write to the issues table. Easiest path: short-circuit the evaluator entrypoint to a no-op and add a comment pointing at the tracking issue.
• Do not drop the table, schema, or the IssueRow-style code paths. We want zero churn so the redesign can plug rules back in without a migration.
• Existing rows: clear on next ingest cycle (or one-time wipe at startup). Document the choice in the PR.

HTTP API

• /v1/issues (or whatever the current endpoint is — list_active_issues route) should return:

  {
    "count": 0,
    "issues": [],
    "status": "placeholder",
    "note": "Issue rules are being redesigned. See tracking issue #<TRACKING>. No issues will be reported until the new definitions land.",
    "computed_at": "<iso8601>"
  }

MCP tool

• list_active_issues MCP tool returns the same envelope. The note field is the important part — AI consumers will see it and know not to treat absence of issues as "all clear."

Dashboard

• The Issues card stays in the layout but its body is replaced with a single muted paragraph that mirrors the API note text, e.g.:

  Issue detection is paused. The previous rules were retired pending a redesign — see tracking issue #<N>. Use the topology, partitions, and stale-links views to assess network state in the meantime.

• No counts, no severities, no list — just the placeholder text. Keep the card visible so users aren't confused by a missing section between releases.

Out of scope

• Designing the new issue rules. That work lives in the tracking issue.
• Schema or table changes.
• Removing the issues column/badge from any other view (node detail, etc.) — replace those with the same placeholder text if they reference issues directly.

Acceptance

• gh api / curl against /v1/issues returns the placeholder envelope with status: "placeholder" and count: 0.
• Dashboard shows the placeholder text in the Issues card.
• MCP list_active_issues returns the placeholder envelope.
• No rows are written to the issues table during a full ingest cycle.
• No regression in any other view.