pr_merge was an independent merge path with no CI, protection or tier check. It now reads the PR, the base's
branch protection and the head commit's CI runs, takes the tier from the issue named in `issue`, and runs the same
evaluateShipGate as tbd_ship (helpers shared, not copied). A refusal is a result (merged:false, reason, ci_status,
risk); an unreadable PR or protection probe is an error; a merge conflict is still an error. Above risk/low a human
merges in the UI. Tests: tier table, no-issue, issue lookup failures, CI and protection not overridden by risk/low,
read-side failures, tier read from the named issue only; nine wiring mutants flip-tested red. Shared reason text now
says "PR left open" (the PR already exists for pr_merge).
Behaviour change: pr_merge without `issue`, or on a PR with no green CI, merges nothing.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SZkhi49hfeSqBp5aZJNXsM
mathiasandClaude Sonnet 5.5
c3d8401604