feat(plans): save assistant messages as editable project plans with improve/implement flows

Add Save-as-plan action on assistant messages that writes a markdown plan
into the project plans directory and suggests a title from the text. Plan
view opens a specific saved plan via targetPath, supports inline edits, and
can kick off improve or implement flows in a new or worktree session.

Ship the matching hidden magic prompts (plan.todo/improve/implement) so the
instructions applied by these flows are visible and editable in the Magic
Prompts page.

Remove the plan editor font override, CodeMirror now uses the app font stack.

packages/ui/src/components/sections/magic-prompts/MagicPromptsPage.tsx

@@ -100,6 +100,30 @@ const PROMPT_PAGE_MAP: Record<string, PromptPageConfig> = {
       { id: 'git.integrate.cherrypick.resolve.instructions', title: 'Instructions' },
     ],
   },
+  'plan.improve': {
+    title: 'Improve Plan',
+    description: 'Hidden prompt used when sending a saved plan into an improve flow.',
+    blocks: [
+      { id: 'plan.improve.visible', title: 'Visible Prompt' },
+      { id: 'plan.improve.instructions', title: 'Instructions' },
+    ],
+  },
+  'plan.todo': {
+    title: 'Todo Planning',
+    description: 'Hidden prompt used when sending a todo into a new planning session.',
+    blocks: [
+      { id: 'plan.todo.visible', title: 'Visible Prompt' },
+      { id: 'plan.todo.instructions', title: 'Instructions' },
+    ],
+  },
+  'plan.implement': {
+    title: 'Implement Plan',
+    description: 'Hidden prompt used when sending a saved plan into an implement flow.',
+    blocks: [
+      { id: 'plan.implement.visible', title: 'Visible Prompt' },
+      { id: 'plan.implement.instructions', title: 'Instructions' },
+    ],
+  },
 };
 
 const hasOwn = (input: Record<string, string>, key: string) => Object.prototype.hasOwnProperty.call(input, key);

packages/ui/src/components/sections/magic-prompts/MagicPromptsSidebar.tsx

@@ -32,6 +32,14 @@ export const MagicPromptsSidebar: React.FC<MagicPromptsSidebarProps> = ({ onItem
           { id: 'github.pr.comment.single', title: 'Single PR Comment Review' },
         ],
       },
+      {
+        group: 'Planning',
+        items: [
+          { id: 'plan.todo', title: 'Todo Planning' },
+          { id: 'plan.improve', title: 'Improve Plan' },
+          { id: 'plan.implement', title: 'Implement Plan' },
+        ],
+      },
     ] as const;
   }, []);