Backlog priority: priority:* labels drive census ordering

[robercano-ghbot]

What

The census currently picks the lowest-numbered planned issue — priority cannot be expressed without renumbering. The four priority labels now exist (priority:critical/high/medium/low, created 2026-07-21).

Spec

• loop-census.sh candidate selection sorts by (priority rank, issue number): critical < high < medium < low < UNLABELED (explicit priority always outranks none; number remains the tiebreaker, so a label-free backlog behaves exactly as today).
• Blocking-graph edges still override order (a blocked candidate is skipped regardless of priority) — priority is preference, edges are semantics.
• Cockpit: show a priority chip on queue/census rows.
• docs/USAGE.md: document the scheme (owner sets labels in the GitHub UI; nothing else changes).

Acceptance

• Census picks priority:critical over lower priorities regardless of issue number; ties break by number; unlabeled ranks last.
• loop-census.test.sh covers: priority beats number, unlabeled-last, blocked-high-priority skipped for unblocked-lower.
• Cockpit renders the chip; USAGE.md updated.

🤖 Generated with Claude Code

[robercano-ghbot]

Issue #173's feat/issue-173-* branch has stalled and already been resumed 2 times without landing a PR. The loop will not retry it automatically -- labeling needs-human. Address it by hand, then either close it out or clear its entry in .claude/state/loop-resume-attempts.json to let the loop resume.