fix(studio): review round 1 for imported-course editing

- Media exclusions move from the editable root .gitignore to the repo's
  info/exclude, and dirtyFiles never counts files listed in media.json, so an
  author's or remote's .gitignore cannot block publish or Sync. Publish and
  Sync refuse commits that track a file over referenced media.
- Importer jobs (code-repo, pdf-deck, transcribe) and the cover route run
  under the course lock now shared with the studio (core withCourseLock) and
  re-check the course inside it; after conversion they fail with "this course
  is now edited in the studio". Convert answers 409 while an import job for
  the course is queued or running, and the job re-checks.
- Crash-safe conversion: the import folder is hidden (.converting-<id>) before
  the published worktree exists; cross-filesystem moves copy into a staging
  dir, fsync, verify size and sha256, then rename; the source is retired as
  .converted-<id>.<n>. Recovery runs at server start and before every convert;
  a re-run of a finished conversion reports success. The loader skips
  dot-folders.
- Minor: no raw NUL bytes in source, link fallback only on EXDEV/EPERM/EMLINK,
  referenced media are not read whole by the editor, missing media are logged,
  mediaDir JSDoc, docs on per-author copies across filesystems.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/feature-import/src/context.ts

@@ -1,6 +1,7 @@
 import { mkdir, readdir, rename, stat } from "node:fs/promises";
 import { join, relative, resolve } from "node:path";
 import type { ServerContext } from "@stc/contracts";
+import { withCourseLock } from "@stc/core";
 import { readRawManifest, writeManifest, type RawManifest } from "./util/manifest";
 import { slugify, uniqueId } from "./util/text";
 import { extractFrame, type Tools } from "./media";
@@ -65,9 +66,21 @@ export async function publishCourse(d: Deps, stage: string, id: string): Promise
 
 export interface ManagedCourse { id: string; dir: string; manifest: RawManifest }
 
+export const STUDIO_OWNED = "this course is now edited in the studio; add files there instead";
+
+/**
+ * Runs importer work that changes an existing course under the course lock shared with the studio (core
+ * `withCourseLock`), after checking, inside the lock, that the course is still an importer folder. A conversion to a
+ * studio course therefore either waits for the job, or the job fails cleanly with STUDIO_OWNED and writes nothing.
+ */
+export function withManagedCourse<T>(d: Deps, courseId: string | undefined, fn: (c: ManagedCourse) => Promise<T>): Promise<T> {
+  return withCourseLock(courseId ?? "", async () => fn(await managedCourse(d, courseId)));
+}
+
 /** An existing course the importer may add to. Only folders directly under <dataDir>/courses qualify: overlays and authored repos are never edited. */
 export async function managedCourse(d: Deps, courseId: string | undefined): Promise<ManagedCourse> {
   if (!courseId) throw new Error("options.courseId is required: choose the course to add to");
+  if (isPlainId(courseId) && (await exists(studioRepo(d, courseId)))) throw new Error(STUDIO_OWNED);
   const dir = join(coursesDir(d), courseId);
   const rel = relative(coursesDir(d), dir);
   if (!isPlainId(courseId) || rel.startsWith("..") || !(await exists(join(dir, "stcourse.yaml")))) {