docs: revise per review — no specific example, gate is per-repo config, soften auto-merge framing

- Drop the concrete incident reference; the doc should stand on its
  own without pointing at one PR.
- Reframe the three-part gate as per-repo branch-protection settings,
  not universal fact — CI-required, stale-review-dismissal, and
  auto-merge are each configurable, not hive law.
- Reframe what auto-merge signals: the arming human already judged
  the PR sound at a coarse level and expects only minor tweaks past
  that point, not zero human review ever. A reviewer's bar is
  substantive doubt, not any nit.

CLAUDE.md

@@ -231,6 +231,9 @@ read them à la carte.
 - **"How do I export Claude Code metrics (tokens, cost, tool calls) to
   a Prometheus/Grafana collector? What OTEL options are available?"** →
   [`docs/observability.md`](docs/observability.md).
+- **"What does a PR review verdict actually gate? Should a reviewer
+  wait for CI before approving? What does arming auto-merge commit an
+  operator to?"** → [`docs/pr-review-gate.md`](docs/pr-review-gate.md).
 
 ## Conventions & process
 

docs/pr-review-gate.md

@@ -0,0 +1,65 @@
+# The PR review gate
+
+What a review verdict means, why a reviewer shouldn't wait on CI to
+submit one, and what "armed to auto-merge" actually signals about the
+human review that already happened.
+
+## The gate has (up to) three parts, and they're per-repo settings
+
+Whether a PR can merge, and what counts toward "can", is configured
+per repo in its branch-protection settings — not a fact true of every
+hive or every repo. The pieces a repo *can* require:
+
+- **CI is green** — the repo's required status checks pass on the
+  PR's current head commit, if the repo requires any.
+- **Requested reviews are `APPROVED`** — and whether a review is
+  invalidated by a later commit ("stale") is itself a setting
+  (Forgejo's "dismiss stale approvals" branch-protection option), not
+  universal behavior.
+- **Someone with write access has armed the PR to merge** — a manual
+  merge once the required conditions hold, or Forgejo's auto-merge
+  (merges automatically the moment the other required conditions are
+  met).
+
+Where these are required, they're independent of each other. A
+reviewer only ever owns the review-approval piece — CI resolves (or
+doesn't) on its own regardless of what a review says, and merge-arming
+is someone else's call.
+
+## Reviewers: submit the verdict, don't gate it on CI
+
+Submit `hive-forge pr-reviews <pr> --approve` or `--request-changes`
+as soon as you've finished checking the diff — don't hold it back
+waiting for CI to go green first. Mention CI's current state in the
+review body if it's relevant (e.g. "approving; `nix flake check` is
+still running"), but don't gate the formal verdict on it: CI isn't a
+signal a reviewer waits on, it's a separate condition that resolves
+independently.
+
+## What arming auto-merge actually means
+
+Auto-merge isn't "no human ever looked at this." Whoever arms it has
+already judged the PR sound at a coarse level — the signal it sends is
+roughly *"apart from maybe minor tweaks a reviewer can still catch,
+I think this is fine."* That's the human-in-the-loop step, and it
+already happened. No large changes are expected to surface after
+that point — a reviewer's job past that point is to flag it if one
+does, not to assume none ever will.
+
+The practical consequence for a reviewer: on a repo where auto-merge
+may already be armed before your review lands, a plain `APPROVED` can
+be the last step before the merge actually happens, with no further
+review pass after yours. That's a reason to actually finish checking
+before approving — not a reason to hesitate over every small thing.
+Genuine, substantive doubt (a claim you haven't verified, a real
+correctness question) is worth a `request-changes` or a clarifying
+comment before approving; a stray style nit isn't the same category.
+
+## Re-review after a repo requires it
+
+Where a repo dismisses stale approvals on a new commit, a review you
+already gave stops counting the moment a follow-up commit lands — even
+one whose message reads as trivial ("just a wording fix", "just
+trimming comments"). Re-diff and re-verify before submitting a fresh
+verdict; don't take a small-sounding commit message as an accurate
+description of the diff.