Silent in-session de-dup of already-seen / already-skipped PRs

If a PR was already inspected or skipped earlier in the same triage session — e.g. it was updated by the author mid-triage and reappears in a later page of results — **skip it silently**. No "you already saw this" prompt; just don't re-process.

**Why:** these reappearances generate noise and trick the operator into re-doing the same decision, sometimes with a different outcome from the first pass.

### Acceptance
- Maintain an in-session set of PR IDs already touched (any terminal classification: applied, skipped, deferred, auto-skipped).
- On re-encounter within the same session: drop without surfacing in the queue or in the progress output.
- The set lives only for the session — a fresh session starts empty.