Mark tasks carrying an artifact or a link on the dashboard

[tildesrc]

Scanning the task table told you a task's state and whose turn it was, but not whether it had a plan to read or a PR to open — you had to press a or p on each row to find out.

This adds a two-slot marks column, left of the name it annotates: ❏ when the task has an unhidden artifact, ➚ when it has a url. The header doubles as the legend.

  state          turn       container   repo       ❏➚  slug[memo]
  ITERATING      agent      live        web-api    ❏➚  add-oauth[Add OAuth login]
  PLANNING       user       live        web-api    ❏   fix-upload[Flaky S3 upload]
  MERGING        agent      starting    dashboard   ➚  dark-mode[Dark-mode theme]
  ORCHESTRATING  agent      live        infra          q3-cleanup[Q3 tech-debt]

Getting the bit to the dashboard

Artifacts are files rather than task columns, so presence has to ride along on the list response or the dashboard would need a request per row on every refresh. GET /tasks now reports has_artifacts, resolved once per response through a new bulk ArtifactStore.tasks_with_artifacts. It's concrete on the ABC — defaulting to list() per id, the way link_slug defaults to a no-op — so no adapter breaks by not knowing about it, and the filesystem store overrides it with a single-pass os.scandir that stops at the first unhidden entry.

Writing an artifact now bumps the change feed too. An artifact stores no row of its own, so without that a newly written plan.md wouldn't wake a parked long-poll and the mark would lag until some unrelated mutation came along.

The dotfile rule that decides "unhidden" moves to core.artifacts.is_hidden, shared with the dashboard's existing "Show hidden" toggle rather than spelled out at each surface.

Why these glyphs

❏ (U+274F) and ➚ (U+279A) rather than 📁/🔗: both are East_Asian_Width=Neutral with no emoji presentation form, so they occupy exactly one cell in every terminal. Emoji are Width=Wide, which makes the rendered column width depend on the viewer's terminal and font. A test pins equal cell width across all four mark combinations, so a later edit can't quietly regress it.

Notes for the reviewer

• has_artifacts is on the summary shape only, not TaskOut. _task_out is sync and called from ~20 mutation handlers; making it async to add a listdir per mutation response isn't worth it for a field only the table needs, and GET /tasks/{id}/artifacts already answers it exactly for one task.
• Tests that read row cells positionally now resolve the index by column label, so inserting a column doesn't renumber assertions across the file.
• Pre-existing and left alone: the table already renders Ambiguous-width glyphs (the │ ├─ └─ connectors, the – status placeholder, the snoozed ·), which do drift in a CJK-configured terminal. Independent of this change and worth its own decision.

Plan: the task's plan.md artifact.