Add plugin section icons and sidebar actions

[brsbl]

What was wrong

Plugins could not express thread-section presentation or native header actions through supported app and SDK boundaries. Section icons were not persisted or returned by the public API, and a plugin action could not ask Core to require an intentional primary-modifier activation, so consumers either duplicated sidebar UI or accepted accidental clicks.

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.
• Add the optional experimental experimental_requiresPrimaryModifier registration flag. Core keeps plain click inert for opted-in actions and accepts Command/Ctrl-click plus Command/Ctrl+Enter/Space.
• Render configured section icons in native sidebar rows and carry the icon field through bootstrap, storage, migration, CLI, and test harnesses.
• Leave the inherited PLUGIN_SDK_VERSION unchanged at 0.4.14; this layer does not restamp or relabel an existing SDK release.

Before — exact merge base f54b2f84b

After — exact head 058027f52

How you verified

• Node v22.22.1 arm64 with Turbo typechecks for @bb/app, @get-bb/plugin-sdk, and @bb/server (7/7 tasks).
• Exact-head focused tests: app sidebar/activation interactions 11/11; plugin SDK app harness 46/46; plugin-authoring documentation contract 15/15.
• Layer-specific thread-section validation also covers the public route, CLI output, SDK, database lifecycle, and generated migration chain.
• git diff --check f54b2f84be86ea23449031cf9614170c929a9b91..058027f527333ad9b9cd3e0077443bd2a1c26257.
• Desktop visual QA used the exact merge base and exact head with the same Delivery / Release checklist fixture, / route, open Delivery actions menu, and 1224×768 native window bounds.

Fixes # — no linked issue.

BB-Thread-ID: thr_eibej5m9xw

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