Fix Pi process notification turns

[ryanbbrown]

Depends on #1663. This branch is stacked on that focused agent_end fix; commit 53ae25671 contains the process-notification follow-up.

What changed

• translate Pi ad-process:notification messages into visible turn input
• start or reuse one projected turn for attention: "turn"
• ignore matching message_end envelopes to prevent duplicate output
• keep idle context and all ignore notifications from opening or changing turns
• keep process notification item IDs distinct across resumed sessions
• show a warning when a process-triggered run returns no assistant text
• avoid classifying an accepted user prompt as a process-triggered turn

Root cause

Pi already invoked the provider for the recorded notification. bb did not recognize the custom SDK message envelopes, so it stored them as provider/unhandled. Pi could then finish with no projected input and no visible assistant output.

Fixes #1681

Validation

• pnpm exec turbo run test --filter=@bb/agent-runtime --force
  • 29 test files passed
  • 323 tests passed
• pnpm exec turbo run typecheck --filter=@bb/agent-runtime --force
• Prettier and git diff --check passed
• independent review found no blockers after two lifecycle findings were fixed

A live Pi extension run was not repeated after porting this change to the new event translator.

AGENT GENERATED: by GPT-5.6 Sol