[Feat] Run multiple Fast tasks in one conversation

[roomote-roomote]

​Opened on behalf of @mrubens. Follow up by mentioning @roomote-roomote or in the web UI.

What changed

• Fast conversations now resolve and render every active delegated task, using each task's newest run so stale restart history cannot revive settled work.
• Task messages and cancellations target an explicit active task when several are running, while preserving implicit routing when exactly one task is active.
• Independent repository work can launch without waiting for existing tasks, and Slack, Discord, and delegated-task lifecycle events share the plural task context. Discord child-thread launches and snapshot resumes retain their parent Fast session.
• Added focused coverage for second launches, task-specific messaging and cancellation, ambiguous requests, concurrent lifecycle state, and zero/one-task compatibility.

Why this change was made

Fast mode previously exposed one active task per conversation and blocked unrelated work until it settled. That also made lifecycle updates vulnerable to clearing or retaining the wrong task when several delegated runs shared a thread.

Impact

Users can run multiple independent Fast delegations in one conversation and continue or cancel the intended task without disrupting the others. Ambiguous follow-ups now produce a clarification instead of being routed arbitrarily.

[roomote-community]

No new code issues found. See task

• Discord Fast task lookup loses sibling tasks after their launches create child threads (apps/api/src/handlers/discord/index.ts:779).
• Snapshot-resumed Discord Fast tasks lose their parent session and disappear from Fast task controls (packages/types/src/task-runs.ts:1052).

_{Reviewed 6b76309}

[daniel-lxs]

Validated locally: multi-task session linkage, plural active-task lookup, and explicit task cancellation all pass.