docs: mergeable is a plain bool, not a tri-state on the wire

AGENTS.md

@@ -222,10 +222,20 @@ wrapped per stage (login / read denied / write denied) via `ErrVaultDenied`.
   worktree root.
 - `watchpr` alerts on changes, not on conditions: the baseline snapshot is a real
   read, so a PR that is already conflicted or already CI-failing when watching
-  starts is polled on rather than reported. Mergeability is the one rule needing
-  a run of observations (`prWatch`), because a conflict must both persist for two
-  polls and follow a mergeable one; Gitea's `mergeable` is tri-state (absent/null
-  = not yet computed) and unknown counts as neither answer.
+  starts is polled on rather than reported. That suppression is printed with the
+  baseline, since a silent watcher and a watcher with nothing to say look alike.
+- Mergeability is the one `watchpr` rule needing a run of observations
+  (`prWatch`). Gitea 1.26 sends `mergeable` as a plain bool — always present,
+  never null — and sends `false` both for a real conflict and while it recomputes
+  the merge base after a push, which is exactly when an agent starts a watch. So
+  `false` counts only once this watch has seen a merge computation start (the PR
+  was mergeable, or `head.sha`/`base.sha` moved), and then only across two
+  consecutive polls of an unchanged head and base; a mergeable poll, an unknown
+  one or a failed poll all break the run. `base.sha` is the base branch tip as of
+  the response, not the merge base at PR creation, so it moves when main does.
+  `Mergeability` stays tri-state for what the bool cannot carry: an absent/null
+  flag from some other Gitea, and a state no successful poll ever filled in.
+  Head and base SHAs are read only to arm this rule — a push never alerts.
 - CI "combined status" comes from `/commits/{sha}/status`; an empty head SHA
   yields an empty state without an API call.
 - Gitea backs every PR with an issue of the same number and serves comments from

README.md

@@ -88,7 +88,13 @@ gets a **new comment from someone other than the agent**, its **CI fails**
 transitions — CI `pending`→`success`, the agent's own comments and pushes — are
 ignored, and so is any condition the PR was already in at the baseline: a PR
 that is already conflicted or already failing when watching starts keeps being
-watched.
+watched, and the baseline line says which conditions it is staying silent about.
+
+Gitea reports `mergeable: false` both for a real conflict and while it
+recomputes the merge base after a push, so a conflict is only reported once the
+watch has seen a merge computation start — the PR was mergeable, or its head or
+base commit moved — and then only across two consecutive polls of an unchanged
+head and base.
 
 ```bash
 # Watch until something meaningful happens (default interval 60s)