Open a task's artifact folder from the a modal with f

[tildesrc]

The repo-artifact modal (A) has had f — open the repo's artifact folder in the host's file manager — since repo artifacts landed. The task-artifact modal (a) had no equivalent: a task's artifacts could only be opened one file at a time, over REST (Enter) or in place (e).

a now reads the same as A:

a            → artifacts modal
  enter      → open the selected artifact over REST
  e          → open the on-disk file in place
  f          → open the folder                      ← new
  ctrl+a     → attach files

• FilesystemArtifactStore.task_artifact_dir — path()'s directory twin, returning <artifacts>/tasks/<task-id>/ or None when it isn't here, so the layout stays owned by the store. It's the id-named directory, not the tasks/<slug> alias symlink.
• action_open_folder moves into the shared _ArtifactListScreen base now that both modals bind it; the Dashboard decides which scope's directory the "folder" mode resolves to.
• The warning differs from the repo modal's, deliberately. A task's directory is created with its first artifact, so "nothing written yet" is the ordinary reason there's no folder and is said plainly, rather than folded into the repo modal's single "isn't on this machine". The folder is not mkdir-ed on demand — ctrl+a already covers adding files.

The key lives in the modal rather than on the task table (where w already opens the task's workdir), keeping the two artifact modals identical.

Docs updated: docs/dashboard.md, docs/tasks.md, the dashboard module docstring, and the AGENTS.md / ARCHITECTURE.md lines that said only repos had a folder key.

Plan: the task's plan.md artifact (panopticon://tasks/de75ab7fd77e4c0d8623590174a6a369/artifacts/plan.md).

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