Error handling

Goal

Recoverable failure is Result. Panics are bugs, not control flow.

Inputs / outputs

• In: a fallible operation
• Out: Result<T, E> with a useful E, or a typed panic only for invariants

Reads / writes

• Read: the fallible fn and its callers
• Write: none unless a playbook opened the task

Approval

Adding thiserror / anyhow if they are not already in Cargo.toml.