fix(agentstack): assignTask refuses tasks in a terminal status

[quappefeeder]

Fixes #46.

assignTask now rejects tasks whose status is terminal (complete/failed/cancelled), reusing the existing TERMINAL set so it matches updateTaskStatus's guard. This prevents silent reassignment of finished/paid tasks and the spurious task.assigned event that followed.

Adds a regression test covering the terminal-assign rejection and asserting the terminal task is left untouched.

Verification:

• git diff --check
• node --check packages/agentstack/src/index.ts
• Full npm/vitest workspace test was not run locally because this Codex Windows environment has Node but no npm/pnpm/yarn binary available.

via ugig gig, ugig user liudong-design