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/settings_view/mod.rs

@@ -272,6 +272,7 @@ pub enum SettingsSection {
     ThirdPartyCLIAgents,
     Bedrock,
     OpenAI,
+    Experiments,
     /// Internal backing-page identifier for CodeSettingsPageView. Multiple subpages
     /// (CodeIndexing, EditorAndCodeReview) share this single backing page,
     /// so this variant is needed as the key in `settings_pages`.
@@ -306,6 +307,7 @@ impl Display for SettingsSection {
             SettingsSection::ThirdPartyCLIAgents => write!(f, "Third party CLI agents"),
             SettingsSection::Bedrock => write!(f, "AWS Bedrock"),
             SettingsSection::OpenAI => write!(f, "OpenAI / LiteLLM"),
+            SettingsSection::Experiments => write!(f, "Experiments"),
             SettingsSection::Warpify => write!(f, "Wormhole"),
             SettingsSection::CodeIndexing => write!(f, "Indexing and projects"),
             SettingsSection::EditorAndCodeReview => write!(f, "Editor and Code Review"),
@@ -363,6 +365,7 @@ impl SettingsSection {
             Self::ThirdPartyCLIAgents,
             Self::Bedrock,
             Self::OpenAI,
+            Self::Experiments,
         ]
     }
 
@@ -405,6 +408,7 @@ impl FromStr for SettingsSection {
             "CloudEnvironments" | "Environments" => Ok(Self::CloudEnvironments),
             "OzCloudAPIKeys" => Ok(Self::OzCloudAPIKeys),
             "SharedBlocks" | "Shared Blocks" => Ok(Self::SharedBlocks),
+            "Experiments" => Ok(Self::Experiments),
             _ => Err(()),
         }
     }