TurnResult: remove text field, simplify oneshot loop

- Remove TurnResult.text (was dead code - Agent::turn handles text internally)
- Simplify run_with_backend to just iterate over steps (Agent::turn loops
  for tool calls and handles empty responses internally)
- Change run/run_shared/run_forked_shared to return Result<(), String>
- Remove AgentResult.output field (no callers used it)
- Stub out legacy text-parsing code (audit, compare) that needs redesign
- Update digest.rs to not depend on text return
- Add level parameter to journal_new/journal_update for digest support

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

Cargo.lock

@@ -555,7 +555,6 @@ dependencies = [
  "serde",
  "serde_json",
  "serde_urlencoded",
- "skillratings",
  "tokenizers",
  "tokio",
  "tokio-rustls",
@@ -2802,12 +2801,6 @@ version = "1.0.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "b2aa850e253778c88a04c3d7323b043aeda9d3e30d5971937c1855769763678e"
 
-[[package]]
-name = "skillratings"
-version = "0.28.2"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "1a6ee7559737c1adcd9184f168a04dc360c84878907c3ecc5c33c2320be1d47a"
-
 [[package]]
 name = "slab"
 version = "0.4.12"