editor: New thread opens the what-to-build screen, never an empty thread

[sandersonstabo]

Why

Clicking New thread landed on an empty thread titled "New task" that said "No messages yet." instead of the "What should we build in editor?" screen. begin_new_task sent CreateTask, and mounting the created thread navigated the new-thread route to it.

What

• begin_new_task no longer creates a thread. It retires the open thread (subscription stop), binds the composer to the project's new-task draft (ComposerDraftScope::Project) and shows the new-thread screen. The Forge creates the thread when the draft is sent, using the existing project-draft send flow (#83457dbf).
• Typed text still carries into the new task. An empty composer shows the project's saved draft instead of overwriting it.
• Choosing another project with an unsent prompt keeps the destination listed as empty until its first send, so no remembered thread opens over the prompt.
• Removed the "No messages yet" overlay, its selector, its predicate and the unused padding constant.

Tests

Rewrote the tests that asserted CreateTask (new-thread retirement, project choice from new-thread/existing/home, initially empty project, desktop new task). They now assert: no thread is created, the source stops, the route is NewThread on the project draft, the draft is kept, and Send is available.

cargo test -p artisan-frontend --lib: 776 passed, 2 failed. work_group_header_row_paints_without_a_disclosure_wrapper and send_has_stable_identity… fail identically on master without this change.

🤖 Generated with Claude Code

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Summary by cubic

Clicking New thread now opens the "What should we build in editor?" screen instead of an empty thread that showed "No messages yet." No thread is created up front; the Forge creates it when the draft is sent, and the currently open thread is retired first.

• Typed text still follows into the new task, and an empty composer shows the project's saved draft instead of replacing it.
• Choosing another project with an unsent prompt keeps the destination listed as empty until its first send, so no remembered thread opens over the prompt.
• Removed the "No messages yet" overlay and its supporting code.
• Tests rewritten to assert no thread is created when opening the new-thread screen and that the project's new-task draft is shown.

^{Written for commit e221050. Summary will update on new commits.}