feat(sessions): follow a provider-originated session switch (Claude /clear, OpenCode /new) instead of quarantining it #894

What happens

A provider can move to a new session from inside its own TUI:

• Claude Code: /clear or /resume.
• OpenCode: /new, /sessions or fork.

Agent Code panes stay bound to the provider session they were launched with. There is no way to follow a provider-originated session change:

• Claude / Codex: the new session's committed entries hit the Claude JSONL transcript can disconnect while proxy/semantic stream stays live, breaking resume identity #290 quarantine (decideJsonlProviderBurst / applyJsonlProviderSessionId in useIpcSubscriptions.ts). The pane shows "Ignored JSONL from provider session X because this pane is bound to Y".
• OpenCode Terminal: since feat(opencode): OpenCode Terminal reports status, turns, conditions and history through opencode-terminal-headless #882 the switch is detected and shown as a pane error, but status, transcript, MCP reads and resume stay on the old session.

Why

Nothing in the session contract can say "the provider itself moved to session Y". Today a different provider session id is indistinguishable from a misattach, and #290 deliberately refuses those.

Suggested direction

• Add an explicit, provider-originated provider-session-changed { from, to, locator } event to the AgentSession contract. Only an adapter that can prove the change from the provider's own signals emits it.
• In main, update the resume id and the transcript locator.
• In the renderer:
  • rebind meta.providerSessionId with a new source;
  • reset the per-session JSONL provider stream state;
  • clear or replace the entries;
  • load history for the new session.
• Keep Claude JSONL transcript can disconnect while proxy/semantic stream stays live, breaking resume identity #290's quarantine for everything else.
• For OpenCode, the package would also rebind its durable reader and live projector to the new session.

Acceptance

After a switch inside the TUI, a pane's status, transcript, MCP reads, phone view and restart-resume all follow the new session. An unrelated provider id still quarantines.

Refs #864