feat: implement Crosscheck Work experiment

Add the 'Crosscheck Work' experiment to the Agents settings. When enabled,
a reviewer sub-agent is spawned after the main agent finishes a turn (with
no pending tool calls). The reviewer critiques the output using a dedicated
system prompt focused on correctness, simplicity, and code quality. If the
reviewer does not respond with 'LGTM!', its feedback is injected as a
synthetic user query back to the main agent, which must address it. This
loop continues until the reviewer approves or max iterations is reached.

Components:
- Feature flag: CrosscheckWork (enabled in DOGFOOD_FLAGS)
- Settings: agents.experiments.crosscheck_enabled,
  agents.experiments.crosscheck_model_id,
  agents.experiments.crosscheck_max_iterations
- Settings UI: new 'Experiments' subpage under Agents
- Crosscheck module: app/src/ai/crosscheck/ with prompt, reviewer model
- Controller integration: hooks into AfterStreamFinished when no actions
  are queued, triggers reviewer, handles feedback injection
- Provider support: OpenAI-compatible and Bedrock direct invocation
- Safety: max iteration guard, error handling, reset on new user query

app/src/ai/mod.rs

@@ -22,6 +22,7 @@ pub mod bedrock;
 pub(crate) mod bedrock_credentials;
 pub(crate) mod block_context;
 pub(crate) mod blocklist;
+pub(crate) mod crosscheck;
 #[cfg(any(feature = "local_fs", not(target_family = "wasm")))]
 pub(crate) mod codebase_auto_indexing;
 pub mod control_code_parser;