Add split spotlight toggle

[brsbl]

Split spotlighting is optional, persists across sessions, and is controllable by people and agents.

What changed

• Add an icon-only split-header toggle shown only when multiple splits are open
• Use “Spotlight this split” and “Clear spotlight” with Idea01Icon and LightbulbOffIcon
• Apply dimming immediately across thread and plugin panes and persist the preference
• Add spotlight and clear-spotlight to the SDK and bb thread pane command
• Keep plugin SDK declarations, generated templates, CLI guidance, and configuration docs in sync

Verification

• Focused app tests: 65 passed across split layout, header, navigation, and WebSocket coverage
• Focused CLI tests: 5 passed
• SDK tests: 39 passed
• Focused server tests: 8 passed
• App, CLI, SDK, server, and server-contract typechecks passed
• Template and plugin-registry generator checks passed
• Real-interface QA confirmed hidden single-thread state, both icon/tooltip states, immediate dimming, CLI delivery, and reload persistence
• Two-pass review loop completed with no remaining P0–P2 findings

AGENT GENERATED: by GPT-5

[SawyerHood]

🚨 SLOP COP 🚨 · review

I am the SlopCop. I am now reviewing this pull request for security, code quality, performance, architecture, and end-to-end behavior.

[SawyerHood]

🚨 slopcop/review — Regenerate all downstream icon artifacts

This shared icon map feeds the plugin registry and the generated plugin starter template. The follow-up commit updates packages/plugin-registry/r/icon.json, but it does not update packages/templates/src/generated/plugin-starter-files.generated.ts. Fresh @bb/templates typecheck and test runs fail, and the two related CI jobs fail for the same stale output. Run the repository generators and commit every changed artifact.

[SawyerHood]

🚨 slopcop/review — Resolved in 747f9b9

The new commit updates the generated plugin starter. The new package test job passes. No action remains for this finding.

[SawyerHood]

🚨 SLOP COP 🚨 · review

ELI5: This adds a lightbulb to the active split. It turns split fading on or off and remembers the choice.

Final result: I found no remaining issue.

Resolved during review

• At commit 6404906, fresh template checks failed because a generated starter file was stale.
• Commit 747f9b9 added that generated file while I posted this review.
• The new package test job now passes. I replied to the inline finding with this update.

Security

I found no security issue. This change only adds local UI state and local storage persistence.

Code and architecture

The code reuses createBooleanPreferenceAtom. It keeps the toggle in one component. I found no harmful duplicate or contract change.

Performance

I found no blocking performance issue. The pane count has an eight-pane limit. The preference changes only after a toggle or a storage event.

Validation

• The app typecheck passed at 838f6f1.
• The 49 focused app tests passed at 838f6f1.
• Browser QA passed with two splits. The button moved with focus and persisted both states after reload.
• Fresh template checks reproduced the stale output at 6404906. Commit 747f9b9 supplied the missing output.

This run started at 838f6f1, as the review marker states. I also checked the later generated-file commits.