mission canvas: add Refine button + markdown-rendered description

Adds a Refine button to the left of Refresh + Launch on the mission
detail toolbar (draft-only). Clicking it POSTs to a new endpoint that
calls Gemini 2.5 Flash to rewrite the user's freeform description into
a coherent, sectioned Markdown brief (Objective / Context / Scope /
Constraints / Acceptance Criteria / Open Questions) ready for the
research + coding agents to ingest cleanly.

Backend:
  - crates/cm-api/src/mission_refiner.rs — Gemini call with a
    system prompt that preserves user-provided facts, avoids
    invention, and emits raw markdown (not JSON).
  - POST /api/missions/{id}/refine — draft-only, 400 on empty
    description or non-draft state.
  - cm-db::repo::missions::set_description helper.

Frontend:
  - MarkdownBlock — tiny zero-dep renderer for h1/h2/h3, bullet +
    numbered lists, **bold**, `code`, paragraphs. Deliberately
    small; the refiner emits a bounded subset.
  - MissionCanvas — Refine button (Sparkles icon, secondary style)
    to the left of Refresh; description now renders through
    MarkdownBlock instead of a single <p>. Disabled while
    description is empty or a refine is in flight.
  - lib/api/missions — refineMission client.

frontend/src/components/dashboard/MissionCanvas.tsx

@@ -9,10 +9,11 @@
 // This replaces ResearchCanvas + LoopsCanvas after Slice 9's cutover.
 
 import React, { useCallback, useEffect, useMemo, useState } from "react";
-import { FileText, Play, RefreshCw } from "lucide-react";
+import { FileText, Play, RefreshCw, Sparkles } from "lucide-react";
 
 import {
   getMission,
+  refineMission,
   setMissionStatus,
   type MissionDetail,
   type MissionStatus,
@@ -21,6 +22,7 @@ import {
   type TaskStatus,
   type TemplateKind,
 } from "@/lib/api/missions";
+import { MarkdownBlock } from "./MarkdownBlock";
 
 const mono =
   "ui-monospace, SFMono-Regular, SF Mono, Menlo, Monaco, Consolas, monospace";
@@ -77,6 +79,7 @@ export function MissionCanvas({
   const [error, setError] = useState<string | null>(null);
   const [tab, setTab] = useState<Tab>("overview");
   const [launching, setLaunching] = useState(false);
+  const [refining, setRefining] = useState(false);
 
   const load = useCallback(async () => {
     if (!selectedId) {
@@ -100,6 +103,21 @@ export function MissionCanvas({
     void load();
   }, [load, refreshKey]);
 
+  const refine = useCallback(async () => {
+    if (!mission) return;
+    setRefining(true);
+    setError(null);
+    try {
+      await refineMission(mission.id);
+      onChanged();
+      await load();
+    } catch (e) {
+      setError(e instanceof Error ? e.message : "refine failed");
+    } finally {
+      setRefining(false);
+    }
+  }, [mission, onChanged, load]);
+
   const launch = useCallback(async () => {
     if (!mission) return;
     setLaunching(true);
@@ -198,6 +216,26 @@ export function MissionCanvas({
             {TEMPLATE_LABEL[mission.template_kind] ?? mission.template_kind}
           </span>
           <div style={{ marginLeft: "auto", display: "flex", gap: 6 }}>
+            {mission.status === "draft" && (
+              <button
+                type="button"
+                onClick={refine}
+                disabled={refining || !mission.description?.trim()}
+                title={
+                  mission.description?.trim()
+                    ? "Refine the description into a sectioned brief"
+                    : "Add a description first"
+                }
+                aria-label="Refine"
+                style={{
+                  ...secondaryBtn,
+                  opacity: refining || !mission.description?.trim() ? 0.5 : 1,
+                }}
+              >
+                <Sparkles size={13} style={{ marginRight: 4 }} />
+                {refining ? "Refining…" : "Refine"}
+              </button>
+            )}
             <button
               type="button"
               onClick={load}
@@ -225,9 +263,9 @@ export function MissionCanvas({
         </div>
         <h1 style={{ margin: 0, fontSize: 20, color: "#f3f3f5" }}>{mission.title}</h1>
         {mission.description && (
-          <p style={{ margin: 0, fontSize: 13, color: "#cfcfd5", lineHeight: 1.5 }}>
-            {mission.description}
-          </p>
+          <div style={{ marginTop: 4 }}>
+            <MarkdownBlock source={mission.description} />
+          </div>
         )}
         <div style={{ display: "flex", gap: 4, marginTop: 4 }}>
           {(["overview", "phases", "tasks", "artifacts", "benchmarks"] as Tab[]).map((t) => {