id(world): F11.13 error taxonomy: stable codes plus reasons on the wire #554

What and why

Typed errors with stable codes and reasons on the wire, including the reason schedule.at drops; every CapError documented in the guide. Stable codes exist, but reasons are dropped, so a guest or operator sees bad_args with no hint why. The DoS suite (F12.18) asserts documented codes.

Source

• .opencode/plans/phase-11-foundations.md:51
• .opencode/plans/2026-10-20_phase-11-status.md:23

Current status

Partial. Stable codes exist (the status audit says planned): pkgs/id/src/world_caps.rs:319-346, six CapError codes (cap_denied, unknown_cap, bad_args, failed, actor_denied, outside_ceiling). Reasons are still dropped: world.rs:2646-2648 logs the check_schedule error at debug and returns plain CapError::BadArgs. No error-code list in a guide was found.

Acceptance criteria

• Every CapError carries its stable code and a reason on the wire
• schedule.at returns the check_schedule reason instead of a plain BadArgs (world.rs:2646-2648)
• Every CapError code is listed in the guide
• Tests assert code and reason for each refusal

Size

M

Prerequisites

• id(world): F11.10 split world.rs, world_session.rs and cli.rs (moves only) #540 — D1-010 F11.10 Split world.rs, world_session.rs, cli.rs (moves only)