feat(task): implement fire-and-forget TaskTool with background execution

[edlsh]

Phase 4: Rework TaskTool to schedule background execution (fire-and-forget)

Changes

• TaskTool now returns immediately after scheduling child session work via SessionRunner
• Added toolPartID to Tool.Context for post-return metadata updates
• Parent tool part metadata updated via Session.updatePart() instead of ctx.metadata()
• Subscribes to SessionRunner lifecycle events for cleanup and status updates

Stack

• PR1: #5996 feat/async-subagent-1-runner
• PR2: #6036 feat/async-subagent-2-background
• PR3: #6041 feat/async-subagent-3-centralize
• PR4: feat/async-subagent-4-tasktool (this PR)