feat(session): centralize async session work through SessionRunner

[edlsh]

Routes async session work through the SessionRunner queue (#5887).

Stacked on #6036 — review/merge that first, then rebase this PR.

Changes:

• Task tool now uses SessionRunner.enqueue() for child sessions
• Server routes background prompts through the runner
• Unified job tracking and lifecycle events for all async work
• Additional tests for task tool integration