fix(unassign): base the unassign label set on live GitHub labels instead of the cached row #1366

Base the label set that POST /api/issues/unassign writes on the live GitHub labels instead of the cached Prisma row, so an unassign can no longer silently strip a label that was added on GitHub since the last sync.

Expected files: src/app/api/issues/unassign/route.ts and its test file (src/app/api/issues/unassign/route.test.ts).

Context:

• updateIssueLabels is a FULL-label-set PUT — it replaces all labels. The unassign route computes that set from issue.labels (the Prisma cache) minus the labels of the requested type.
• The codebase already states the rule this breaks, in src/lib/issue-claim.ts: "Only targeted label writes reach GitHub here … Never replace the whole label set — the cached row be behind GitHub, and a full-set write would silently revert every label change made on GitHub since the last sync."
• The sibling assign route (src/app/api/issues/actions/route.ts) already complies: it reads the live label set through getLiveIssueLabels and fails closed with a 503 when GitHub is unreachable.
• The unassign route cannot succeed without GitHub anyway (it PUTs the label set), so a live read costs no availability: a GitHub outage already fails the write.
• The per-issue mutation anchor that fix(ownership): guard the remote agent/* label write in POST /api/issues/unassign #1361 added to this route is the right place to take the live read — the label-set decision must happen while the anchor is held, so the snapshot the PUT applies is the one the anchor saw.

Acceptance:

• The label set the unassign writes is derived from the live GitHub labels (minus the agent/owner labels of the requested type), not from the cached row.
• GitHub unreachable → fail closed: no remote write, no cache write, a failure audit row, and a 503 — the same shape the assign route uses.
• The live read happens inside the per-issue mutation anchor, and the remote write stays inside the fenced write.

Follow-up to #1361 (the anchor/fence for this route), found while reviewing that change.