WorkflowTask registry + abort controllers

[Michaelliv]

Introduce a per-process WorkflowTask registry.

• WorkflowTask: id (ulid), scriptPath, status (running | paused | completed | failed | killed), snapshot, startedAt/endedAt, abortController, consentAccepted
• Workflow tool registers a task on start, updates it via existing snapshot callbacks, and clears/finalizes on end
• Abort signal threads through to runWorkflow / WorkflowAgent (already wired); tasks expose .abort()
• Expose registry to extension code (in-memory only at this stage)

Parent: #1