Add plugin section icons and sidebar actions

[brsbl]

What was wrong

Plugins could not express thread-section presentation or native header actions through the supported app and SDK boundaries. Section icons were not persisted or returned by the public API, and plugins that needed a section-scoped action had no native slot, which forced them to duplicate sidebar behavior outside the owning surface.

What changed

• Persist optional semantic section icons and expose them through the experimental thread-section API, SDK, and bb thread section CLI flags.
• Add the experimental sidebar section-action slot with validated presentation, inline-preferred placement, overflow behavior, pressed state, and automatic fullscreen cleanup when an owning action disappears.
• Render configured section icons in native sidebar rows and carry the icon field through bootstrap, storage, migration, CLI, and test harnesses.
• Keep PLUGIN_SDK_VERSION at 0.4.10; this is additive API work and does not relabel or vendor SDK 0.4.10.

Before — merge base c942421a4

After — exact head 3f020e404

How you verified

• Node 22 with Turbo typechecks for @bb/app, @bb/cli, @bb/server, @bb/db, @bb/sdk, and @get-bb/plugin-sdk (10/10 tasks).
• Focused tests: app sidebar actions 14/14; plugin SDK app harness 44/44; server section/plugin docs 93/93; CLI organization output 6/6; database lifecycle and migrations 95/95; public SDK 50/50.
• git diff --check c942421a454ee4157004274053eec5f36c81d7eb..3f020e4045655721c5cdfb1ad2ab6986b7569720.
• Desktop visual QA from the exact merge base and exact head using the same Delivery / Release checklist fixture, / route, open Delivery actions menu, desktop zoom, and 1224×768 native window bounds. Both durable captures are 2448×1536 Retina screenshots from the compared dev apps.

Fixes # — no linked issue.

BB-Thread-ID: thr_eibej5m9xw

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