feat(team): team enhancements, subagentModel, parallel tool exec

Port the agent-team enhancement layer onto upstream/main so the model
can spin up coordinated sub-agent teams for parallel work.

subagentModel config wiring (fix):
- Add subagent_model field to RuntimeFeatureConfig + RuntimeConfig::subagent_model()
  accessor so the subagentModel setting (already validated by config_validate.rs)
  is now actually read and stored.
- Agent tool's resolve_agent_model falls back to subagentModel from config
  when no explicit model is passed.

Provider namespace separation:
- New 'custom-openai' provider kind with dedicated env vars
  (CLAWCUSTOMOPENAI_API_KEY / CLAWCUSTOMOPENAI_BASE_URL)
- /setup wizard saves kind: 'custom-openai' for option 5
- Bare model name normalized to 'custom/' prefix to avoid proxy 404s
- Sub-agents inherit /setup-saved provider config via inject_config_as_env_fallbacks

Parallel tool execution:
- Override execute_batch to classify read-only tools as parallel-safe
  and run them concurrently via std::thread::scope
- Results return in original model order

Team coordination layer:
- AgentMessage, TaskClaim, TeamStatus tools + shared mailbox directory
- Mode presets (tiny/1x ... mega/6x) + enriched TeamCreate/Agent descriptions
- Background team watcher
- /team slash command: on/off/status/toggle

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

rust/crates/api/src/error.rs

@@ -180,6 +180,49 @@ impl ApiError {
         }
     }
 
+    /// HTTP status code for an `Api` error, if any. Used by callers (e.g. the
+    /// sub-agent provider chain) to decide whether to advance to the next
+    /// configured model — a 404 "model not found" on the primary should fall
+    /// through to the next fallback rather than killing the whole chain.
+    #[must_use]
+    pub fn status_code(&self) -> Option<reqwest::StatusCode> {
+        match self {
+            Self::Api { status, .. } => Some(*status),
+            Self::RetriesExhausted { last_error, .. } => last_error.status_code(),
+            Self::MissingCredentials { .. }
+            | Self::ContextWindowExceeded { .. }
+            | Self::ExpiredOAuthToken
+            | Self::Auth(_)
+            | Self::InvalidApiKeyEnv(_)
+            | Self::Http(_)
+            | Self::Io(_)
+            | Self::Json { .. }
+            | Self::InvalidSseFrame(_)
+            | Self::BackoffOverflow { .. }
+            | Self::RequestBodySizeExceeded { .. } => None,
+        }
+    }
+
+    /// Response body (best-effort) for an `Api` error, if any.
+    #[must_use]
+    pub fn response_body(&self) -> Option<&str> {
+        match self {
+            Self::Api { body, .. } => Some(body.as_str()),
+            Self::RetriesExhausted { last_error, .. } => last_error.response_body(),
+            Self::MissingCredentials { .. }
+            | Self::ContextWindowExceeded { .. }
+            | Self::ExpiredOAuthToken
+            | Self::Auth(_)
+            | Self::InvalidApiKeyEnv(_)
+            | Self::Http(_)
+            | Self::Io(_)
+            | Self::Json { .. }
+            | Self::InvalidSseFrame(_)
+            | Self::BackoffOverflow { .. }
+            | Self::RequestBodySizeExceeded { .. } => None,
+        }
+    }
+
     #[must_use]
     pub fn safe_failure_class(&self) -> &'static str {
         match self {