## Problem
All JSON error payloads had the same three-field envelope:
```json
{"type": "error", "error": "<prose with hint baked in>"}
```
Five distinct error classes were indistinguishable at the schema level:
- missing_credentials (no API key)
- missing_worker_state (no state file)
- session_not_found / session_load_failed
- cli_parse (unrecognized args)
- invalid_model_syntax
Downstream claws had to regex-scrape the prose to route failures.
## Fix
1. **Added `classify_error_kind()`** — prefix/keyword classifier that returns a
snake_case discriminant token for 12 known error classes:
`missing_credentials`, `missing_manifests`, `missing_worker_state`,
`session_not_found`, `session_load_failed`, `no_managed_sessions`,
`cli_parse`, `invalid_model_syntax`, `unsupported_command`,
`unsupported_resumed_command`, `confirmation_required`, `api_http_error`,
plus `unknown` fallback.
2. **Added `split_error_hint()`** — splits multi-line error messages into
(short_reason, optional_hint) so the runbook prose stops being stuffed
into the `error` field.
3. **Extended JSON envelope** at 4 emit sites:
- Main error sink (line ~213)
- Session load failure in resume_session
- Stub command (unsupported_command)
- Unknown resumed command (unsupported_resumed_command)
## New JSON shape
```json
{
"type": "error",
"error": "short reason (first line)",
"kind": "missing_credentials",
"hint": "Hint: export ANTHROPIC_API_KEY..."
}
```
`kind` is always present. `hint` is null when no runbook follows.
`error` now carries only the short reason, not the full multi-line prose.
## Tests
Added 2 new regression tests:
- `classify_error_kind_returns_correct_discriminants` — all 9 known classes + fallback
- `split_error_hint_separates_reason_from_runbook` — with and without hints
All 179 rusty-claude-cli tests pass. Full workspace green.
Closes ROADMAP #77 Phase 1.
9362900b1b