[Feature] Auto-filter sidebar threads by activity per project

[justsml]

Working across projects means scanning one mixed thread list. Busy projects crowd out quieter ones, making it easy to miss work waiting for input.

This proposal gives each project a compact section with independent history controls:

The result: activity, always visible, with history sized to each project.

• Keep running, waiting, pinned, and currently open threads visible.
• Use muted − / + controls to choose how many additional recent threads appear, ordered by last modified.
• Collapse history while keeping active work visible.
• Put waiting threads first and highlight their entire project section.
• Preserve familiar project icons, with status, branch, and provider metadata aligned consistently.

Initial scope: optional web/desktop layout, remembered per-project preferences, and existing thread actions preserved. The same visibility rules should guide mobile.

Status: visual proposal; implementation and runtime validation remain.

I realize some folks seem to propose similar ideas, but none had a visual to (hopefully) explain it better.

Drafted with GPT-6.1-Sol through the Codex harness in T3 Code.