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 plugins had no supported way to contribute a stateful action to a native sidebar section header.

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, direct native-button activation, and automatic single-section-view 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.
• Bump PLUGIN_SDK_VERSION and @get-bb/plugin-sdk together from 0.4.16 to 0.4.17, the first contract version containing these new public APIs.
• Add a persisted-upgrade regression proving a plugin scaffold created against SDK 0.4.16 still installs and loads unchanged under Core 0.4.17.

Before — exact pre-rebase merge base f54b2f84b

After — exact pre-rebase UI head 058027f52

Final-head recapture note: macOS exhausted application memory while launching isolated desktop dev apps for 19eedbb7f and 32252a432. The existing pair remains attached, but this PR is not represented as fully merge-ready until the exact-final-head pair is refreshed.

How you verified

• Node v22.23.1 arm64 with Turbo: @bb/server typecheck passed.
• Node v22.23.1 arm64 with Turbo: all 58 focused plugin-install tests passed, including the persisted SDK 0.4.16 scaffold upgrade regression.
• Exact-head focused app sidebar/action interactions, plugin SDK harness tests, public route/CLI coverage, database lifecycle coverage, and generated migration-chain tests pass.
• .github/workflows/check-plugin-sdk-version.mjs confirms the public SDK surface changed together with the version bump.
• git diff --check 19eedbb7fb4cc8b9bdb0f8fcbfc92a9b90d8bd0b..32252a4328515231bcafe07a336b936a70e8f728.
• The attached desktop pair used the same fixture, route, interaction state, and native window bounds at the exact pre-rebase revisions shown above; final-head recapture remains blocked as noted.

Fixes # — no linked issue.

BB-Thread-ID: thr_eibej5m9xw

AGENT GENERATED: by GPT-5.6-Sol