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/lib/messages/messageText.ts

@@ -11,3 +11,21 @@ export const flattenAssistantTextParts = (parts: Part[]): string => {
     const combined = textParts.join('\n');
     return combined.replace(/\n\s*\n+/g, '\n');
 };
+
+export const suggestPlanTitleFromText = (text: string): string => {
+    const normalized = text
+        .replace(/\r\n?/g, '\n')
+        .split('\n')
+        .map((line) => line.trim())
+        .find((line) => line.length > 0) || 'Plan';
+
+    const cleaned = normalized
+        .replace(/^#+\s*/, '')
+        .replace(/^[-*+]\s+/, '')
+        .replace(/^\d+\.\s+/, '');
+
+    const sentenceMatch = cleaned.match(/(.+?[.!?])(?:\s|$)/);
+    const firstSentence = sentenceMatch?.[1] || cleaned;
+    const compact = firstSentence.replace(/\s+/g, ' ').trim();
+    return compact.length > 160 ? compact.slice(0, 160).trim() : compact || 'Plan';
+};