[Bug] Plan type cannot be corrected after creation through the API

[HamptonMakes]

Describe the bug

A plan's type can be selected through plan_type when the plan is created, but the documented plan update API does not allow changing it afterward. A mistaken classification therefore appears permanent through the API even though plan content, title, tags, visibility, and references remain editable.

This is especially easy for agent-created plans: an agent may choose General, then recognize from the plan-type descriptions that the content should have been Research. The agent instructions correctly explain the distinction, but provide no supported correction workflow.

To Reproduce:

1. Create a plan with POST /api/v1/plans and "plan_type": "general".
2. Determine that the plan should use the Research type.
3. Consult the agent instructions and PATCH /api/v1/plans/:id contract.
4. Observe that plan_type / plan_type_id is not an allowed update field and no dedicated type-change endpoint is documented.

Expected behavior:

Plan owners should be able to correct the type of an existing plan without deleting, archiving, or recreating it. Ideally:

• PATCH /api/v1/plans/:id accepts plan_type (name) or plan_type_id;
• the web UI exposes the same operation;
• the agent instructions document the correction workflow and any effect on default tags;
• the change is represented in plan history/audit metadata.

If plan types are intentionally immutable, creation should require explicit confirmation and the agent instructions should explain why recreation is the only option. However, mutability seems preferable because classification mistakes do not invalidate a plan's content or history.

Supporting Material

The current agent instructions say:

• “When creating a plan, pass plan_type to associate it with a type.”
• plan_type is optional on create.
• The update endpoint's allowed fields are title, visibility, archived, tags, and references.

No plan-type correction operation is documented.

Environment:

• API: CoPlan v1 REST API
• Client: curl/agent workflow
• Date observed: 2026-07-27