feat(pi): make Sideshow status session-aware

[benvinegar]

Summary

• keep Pi's footer quiet until the current conversation is associated with a Sideshow session
• update the displayed session after publish, update, reply, upload, and conversation-tree navigation
• avoid treating explicit read-only list/wait targets as the remembered session
• retain SIDESHOW_TUI_STATUS=0 as a hard opt-out without disabling tools or trace synchronization
• point Pi directly at the JavaScript extension entry so support files are not auto-loaded as extensions

Why draft

This is an exploratory follow-up to #226. The main product question is whether an idle extension should stay quiet or continue showing the configured Sideshow URL. The current implementation hides the idle URL; retaining it may be more useful, so this is a draft for hands-on evaluation rather than a merge-ready decision.

Validation

• npm test — 432 tests passed
• npm run typecheck
• npm run lint
• npm run format:check
• local Pi package load and RPC status smoke tests

This PR description was generated by Pi using GPT-5.6 Sol