Add sidebar Split lifecycle

[brsbl]

What was wrong

The built-in sidebar could start only one new-thread composer. Creating a two-pane drafting workspace required a drag split, new-thread submission had no pane-local replacement path, and draft rows could not reopen their saved slot in a split. That left the primary creation control, per-pane draft lifecycle, and split workspace behavior disconnected.

What changed

• Adds a trailing Columns2 Split action to the sidebar's New thread row. It appears on hover/focus, stays keyboard reachable, is absent on compact viewports, and replaces the current workspace with exactly two fresh blank panes focused on the left.
• Registers thread.split as a Threads command for the command palette and Keyboard settings, with no default keybinding.
• Keeps new-thread submission pane-local: the sent slot is discarded, the other pane is untouched, navigation follows the existing "Navigate to threads on creation" preference, and failed sends retain the composer.
• Extends split-pane draft lifecycle and leave-toast handling, including one toast for a multi-pane replacement leave event.
• Adds Open in split to draft-row action and context menus when the split workspace is available, reopening the draft's existing slot.
• Reuses the existing split-layout persistence contract; there is no host-daemon wire change.
• Advances @get-bb/plugin-sdk to 0.4.23 because the published command-id union now includes thread.split; 0.4.23 is the next version not already present on npm.

How you verified

• Phase 4 focused regressions: 129 tests passed across RootCompose, split lifecycle, draft menus, toast, command, and layout operations.
• Expanded app regressions: 212 tests passed; full @bb/app suite passed 3,440 tests in 441 files (3 skipped).
• Server keybinding coverage: 8/8 passed. Host-daemon: 558 tests in 45 files passed. CLI: 476 tests in 50 files passed.
• Turbo typecheck: 76/76 tasks passed.
• Lint: zero errors; only the repository's existing 183 app and 19 mobile warnings remain.
• Exact-head GitHub CI run 32964455949: all 13 checks passed, including app shards, server, integration, packages, npm version guard, and Ubuntu/macOS package smoke.
• Chrome for Testing 151.0.7922.71: verified hover/focus affordance, Tab focus, exactly two independent panes with left focus, 1024px layout, compact removal, light/dark rendering, hard reload restore, and no new console errors (only existing unused plugin-style preload warnings).
• Safari 26.5.2: verified keyboard access with macOS's Option-Tab control traversal, Enter activation, exactly two panes with one focused, hard reload restore, 700px compact removal, and persisted light/dark/system theme changes.
• Full-repository test attempts reached 68 successful package tasks before reproducible baseline/environment failures outside this diff: provider fork-recording drift in @bb/provider-parity and bb-plugin-provider-claude-code, /tmp vs /private/tmp diagnostics in @bb/plugin-build, and host-load fixed-timeout failures in @bb/agent-runtime/server. Each changed package and focused changed path passed independently.

Before — parent PR head b971bd795

New thread hovered at the same route, fixture, and 1440×1024 viewport; no secondary action exists.

After — PR head 03d59f8b0

The same state reveals the icon-only Split action without adding chrome at rest.

After — two-pane result

Split replaces the workspace with two blank new-thread panes and focuses the left pane.

After — responsive and dark

Fixes: N/A — Updated Sidebar Spec Phase 4 layer.

BB-Thread-ID: thr_2e2gbjx943

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