feat(missions): an operator button to merge a mission's branch into main

`MergePolicy::Never` — the default for anything touching code — has always meant
"do not merge on your own", deferring to a human. There was no way for that human
to say yes: `auto_merge` was reachable only from the paper-harvest path, no
workflow template declares `merge_policy`, and every mission ended at a branch.

`POST /api/missions/{id}/merge` is that yes, with a button on the artifacts tab.
The additive-only gate does NOT apply here, deliberately: an operator reading a
code change is exactly the judgement the policy was holding out for.

What is not waived:

  - the branch comes from the artifact delivery RECORDED, not rebuilt from the
    mission id, and must have `pushed: true`. A phase that never pushed shows no
    button instead of one that cannot work.
  - an empty branch is refused. A button reporting success for merging nothing
    is worse than no button.
  - a conflict refuses, aborts, and leaves the repo clean rather than forcing.

It works in a FRESH CLONE under `_merge/<mission>`, never the mission checkout:
that directory is reaped on a timer after a mission ends, so a merge using it
would succeed right after a run and fail inexplicably an hour later. The clone is
made by the server process, so nothing runs as root and ordinary cleanup works —
unlike the copies in `root_copy`.

`merge_and_push` is split out so the operator path and the automatic path run the
SAME git commands; only the gates differ. A test asserts both call it, that the
operator path does not re-apply the additive gate it exists to bypass, and that
it still refuses an empty branch.

Harness 43/43 across all five recipes before this change, with `_gate`, `_bench`
and `_verify` all at zero.

246 lib tests, 20 binaries, 89 frontend tests, clean build.

frontend/src/components/dashboard/MissionArtifacts.tsx

@@ -11,11 +11,13 @@
 // research phase can leave dozens of documents.
 
 import { useCallback, useState } from "react";
-import { FileText } from "lucide-react";
+import { FileText, GitMerge } from "lucide-react";
 
 import {
   getArtifactContent,
+  mergeMissionBranch,
   type ArtifactContent,
+  type MergeResult,
   type MissionDetail,
 } from "@/lib/api/missions";
 import { MarkdownBlock } from "./MarkdownBlock";
@@ -32,6 +34,8 @@ export function MissionArtifacts({
 }) {
   const missionId = mission.id;
   const artifacts = mission.artifacts;
+  const [merging, setMerging] = useState(false);
+  const [merge, setMerge] = useState<MergeResult | null>(null);
   const [openId, setOpenId] = useState<string | null>(null);
   const [text, setText] = useState<ArtifactContent | null>(null);
   const [loading, setLoading] = useState(false);
@@ -65,6 +69,32 @@ export function MissionArtifacts({
     [missionId, openId],
   );
 
+  // The branch delivery actually pushed, read from the artifact that recorded
+  // it — not rebuilt from the mission id, so a phase that never pushed shows no
+  // button rather than a button that cannot work.
+  const branch = artifacts
+    .map((a) => (a.metadata ?? {}) as Record<string, unknown>)
+    .filter((m) => m.pushed === true)
+    .map((m) => (typeof m.branch === "string" ? m.branch : null))
+    .filter(Boolean)
+    .pop() as string | null;
+
+  const doMerge = async () => {
+    if (merging) return;
+    setMerging(true);
+    setMerge(null);
+    try {
+      setMerge(await mergeMissionBranch(mission.id));
+    } catch (e) {
+      setMerge({
+        merged: false,
+        reason: e instanceof Error ? e.message : "merge request failed",
+      });
+    } finally {
+      setMerging(false);
+    }
+  };
+
   if (artifacts.length === 0) {
     return (
       <div style={{ fontFamily: mono, fontSize: 11.5, color: "#8a8a92", padding: 12 }}>
@@ -75,6 +105,42 @@ export function MissionArtifacts({
 
   return (
     <div style={{ display: "flex", flexDirection: "column", gap: 8 }}>
+      {branch && (
+        <div
+          style={{
+            padding: 11,
+            borderRadius: 10,
+            border: "1px solid rgba(124,214,224,.22)",
+            background: "rgba(124,214,224,.05)",
+            display: "flex",
+            alignItems: "center",
+            gap: 10,
+          }}
+        >
+          <GitMerge size={16} style={{ color: "#7cd6e0", flex: "none" }} />
+          <div style={{ flex: 1, minWidth: 0 }}>
+            <div style={{ fontSize: 13, color: "#f3f3f5", fontWeight: 500 }}>
+              Merge this mission&rsquo;s work
+            </div>
+            <div style={{ fontFamily: mono, fontSize: 10.5, color: "#8a8a92" }}>
+              {merge ? merge.reason : `${branch} → default branch`}
+            </div>
+          </div>
+          <button
+            type="button"
+            onClick={doMerge}
+            disabled={merging || merge?.merged === true}
+            style={{
+              ...secondaryBtn,
+              padding: "5px 10px",
+              fontSize: 11,
+              opacity: merging || merge?.merged ? 0.6 : 1,
+            }}
+          >
+            {merging ? "Merging…" : merge?.merged ? "Merged" : "Merge to main"}
+          </button>
+        </div>
+      )}
       {artifacts.map((a) => {
         const isOpen = openId === a.id;
         return (