Quest prerequisites: block quest start until required quests are completed

[leocagli]

Context

The quest system currently allows any agent to start any quest. This issue adds a prerequisites field to quests, so advanced quests can require earlier ones to be completed first.

What to implement

Quest model change

Add optional prerequisites: string[] — an array of quest IDs that must be in quest.completed state for the agent before they can start this quest.

Validation on POST /api/quests/:id/start

Before marking a quest as started:

1. Look up the quest's prerequisites array
2. For each prerequisite ID, check if the requesting agent has a completed quest with that ID
3. If any prerequisite is missing → return 403 Forbidden with body {ok: false, reason: "prerequisites_not_met", missing: ["quest-id-1"]}

Issue creation

POST /api/quests should accept the new prerequisites field. Circular prerequisites (A requires B, B requires A) must be rejected with 400.

Acceptance criteria

• Creating a quest with prerequisites: ["quest-1"] succeeds
• Starting a quest fails with 403 if prerequisites not completed
• Error body lists which prerequisites are missing
• Circular prerequisites rejected on creation (not on start)
• Quest with no prerequisites starts as before (no regression)
• Unit tests: missing prereq, satisfied prereq, circular detection

🎁 Evidencia visual = reward extra

• Si tu PR incluye video demo o capturas de pantalla mostrando la funcionalidad pedida funcionando end-to-end, va a ser considerado para rewards de GrantFox en esta issue. No es obligatorio, pero suma mucho para la evaluación.

[AbuJulaybeeb]

hi maintainer,
i would like to take on this issue