Built-in task type: self-media (trends -> topic -> draft -> archive) #2714

What was observed

A recurring self-media content task currently has to be configured as a custom task
type: a Markdown file placed in ~/.emrg/task-templates/.

That file is host-local state, not source. It does not ship with a release, does not exist
on the next machine, is displayed by the GUI as a custom type rather than a built-in one,
and no test reads it — so a change to the scheduler's template resolution, or to the
documented type list, can break it silently and nothing goes red.

The eight built-in task types (evolution, paper, open-source, promote, journal,
competition, ios-app, feishu) do not have these faults: their prompts live in
emrg/server/*_prompt.md, ship with the package, are listed by the GUI as read-only
built-ins whose body can be viewed, and are covered by tests/test_scheduler.py and by the
bidirectional task-type/README guard in tests/test_readme_matches_code.py.

What is expected

self-media becomes a ninth built-in task type, on the same footing as the other eight: a
tasks.yml record carrying type: self-media works on any installation with no local
template file, and the prompt it renders ships with the version.

The prompt must run a four-step round and stop there:

1. Read platform trends — graded. Use live heat data where it can actually be read;
   where it cannot, choose from domain common sense and annotate the product as "no
   real-time data was obtained", rather than presenting an assumption as a measurement.
2. Choose a topic — one topic per round, with the source of that choice named.
3. Draft — one of three declared structures (list-style, single-point long form,
   narrative), and the round states why it chose that one.
4. Archive, and record the round's readings — one file per round, in the task's own
   directory, with the date and the target platform in the file name; the file also carries
   the round's readings (topic source, chosen structure, word count).

Out of scope: no publishing, and no platform account login. The round ends at a
drafted, archived product, and the readings it records are not sent anywhere.

Acceptance

• TASK_TEMPLATES maps self-media to a new emrg/server/self_media_prompt.md; the file
  exists and is non-empty. Deleting the mapping, or pointing it at a filename no file has,
  fails a test.
• The template names the four steps above, the three structures, the annotation for a round
  without live data, the archive naming rule, the readings that must be recorded, and the
  no-publish / no-login boundary.
• self-media appears in the built-in task-type list of both README.md and
  README.cn.md — the guard reads both files, in both directions.
• A self-media record passes validate_task_record, and an unknown type is still refused.
• Built-in resolution still wins over a same-named custom template: a self-media.md in
  ~/.emrg/task-templates/ cannot shadow the built-in one. Inverting that precedence fails a
  test.
• list_templates() reports it as builtin: true with a non-empty prompt, which is what
  puts its body in the GUI's read-only viewer.