Move thread search into the command palette

[brsbl]

What was wrong

Thread search lived in the sidebar while the command palette had no reusable mode boundary. Root commands and searchable content were mixed, and thread search disappeared with the sidebar.

What changed

• groups the palette root at rest into Threads, Actions, and Plugins
• switches to one flat relevance-ranked list while typing
• adds a dedicated Threads mode with one unlabelled active → draft → archived list and All / Active / Drafts / Archived scope
• implements keyboard scope selection, focus transfer, one-level Escape, and scope reset
• removes the app-owned sidebar search field while preserving plugin-provided thread-list search contracts

No wire, daemon protocol, CLI, SDK, plugin API, or keybinding contract changes are introduced by this layer.

Visual evidence

Rebase evidence status: current parent 4f7fcae65c029baa8cc91b657251da9dbfaf6947 and current head 307d782217a81f623aa65cf5211ff8504120d2e7. The embedded captures below were made from the exact pre-rebase revisions and remain useful behavioral references, but they are not exact-current-revision evidence. Replace them before merge. The persistent root footer is intentionally not part of this layer; it is added by child PR #2778.

Pre-rebase behavioral comparison: fd1c3ea60b4ffeafefeab4c5bff8b893d8364531 → 4a70ddfa811e1cbc2e9f59be17beb4b1ce84d6f8.

Before — pre-rebase base fd1c3ea60b4ffeafefeab4c5bff8b893d8364531

The palette is an undifferentiated command stream.

After — pre-rebase head 4a70ddfa811e1cbc2e9f59be17beb4b1ce84d6f8

The same root state is grouped by Threads, Actions, and Plugins.

After — flat ranked query

After — Threads mode

How you verified

• Google Chrome for Testing 151.0.7922.71, pre-rebase exact branch web apps from scripts/bb-dev-app, light theme, /, 1440×900.
• At root, observed exactly three buckets; decorative leading icons are absent there. In Threads mode, the only visible leading icon is the magnifier in the Threads mode chip.
• Typed tog and observed one flat ranked result list without bucket headings.
• Used Tab to enter Threads scope, ArrowDown to change All → Active, and Enter to focus search. Escape returned to root, re-entry reset scope to All, and the next Escape closed the palette.
• Confirmed the app-owned sidebar search field was absent. No page or console errors were recorded.

Fixes: N/A — Updated Sidebar command-palette search layer.

BB-Thread-ID: thr_z45dnp4jyw

AGENT GENERATED

[brsbl]

Superseded by the standalone command-palette layer #2851 in the new approved stack.