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/index.css

@@ -66,12 +66,6 @@ textarea[data-chat-input="true"]:focus-visible {
   background: color-mix(in srgb, var(--accent) 70%, transparent);
 }
 
-.oc-plan-editor .cm-scroller,
-.oc-plan-editor .cm-content,
-.oc-plan-editor .cm-gutters {
-  font-family: "IBM Plex Sans", "SF Pro Text", -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif !important;
-}
-
 /* Codemirror syntax fallback for classHighlighter tokens */
 .cm-editor .tok-comment,
 .cm-editor .tok-docComment,