harness-tools + app: task tool, subagent spawner, board wiring (M4)

The `task` tool delegates to a SubagentSpawner (owned by the composition root):
resolves the agent, enforces the depth limit, applies permission intersection,
filters tools per-agent, and runs the child session foreground or background.
Foreground returns the child's final text; background registers on the job board
and detaches under the parent run token. The run loop injects the board into
primary-agent requests and reconciles terminal jobs each step.

Integration tests cover foreground run + alias reuse continuing the same child
session, depth-limit and unknown-agent rejection, and board prompt injection.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/harness-core/src/engine/processor.rs

@@ -10,10 +10,13 @@ use crate::event::{AppEvent, EventBus};
 use crate::llm::{FinishReason, LlmEvent, LlmEventStream, ProviderError};
 use crate::permission::{PermissionService, Ruleset};
 use crate::store::Store;
-use crate::tool::{MetadataSink, PermissionHandle, Tool, ToolCtx, ToolError, ToolRegistry};
+use crate::tool::{
+    MetadataSink, PermissionHandle, SubagentSpawner, Tool, ToolCtx, ToolError, ToolRegistry,
+};
 use crate::types::{Message, MessageId, Part, PartBody, PartId, SessionId, TokenUsage, ToolState};
 
 use super::doomloop::DoomLoopGuard;
+use super::jobs::JobBoard;
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum StepResult {
@@ -47,6 +50,9 @@ pub struct StepContext {
     pub permissions: Arc<PermissionService>,
     pub static_rules: Ruleset,
     pub extra_rules: Arc<Mutex<Ruleset>>,
+    /// Parent-effective ruleset for a subagent session; empty for a root session. Enables
+    /// permission intersection on this session's tool calls.
+    pub parent_rules: Ruleset,
     pub session_id: SessionId,
     pub cwd: PathBuf,
     /// Session's `tool-output` spill directory (see `tool::truncate`).
@@ -54,6 +60,11 @@ pub struct StepContext {
     pub cancel: CancellationToken,
     /// Wall-clock for `created_at` stamps — passed in so tests stay deterministic.
     pub now: i64,
+    /// Lets the `task` tool spawn subagents. `None` disables delegation (headless/tests).
+    pub spawner: Option<Arc<dyn SubagentSpawner>>,
+    /// This session's background job board (as a parent). Injected into requests when the
+    /// running agent can delegate; `None` disables the board.
+    pub job_board: Option<Arc<JobBoard>>,
 }
 
 struct FlushTracker {
@@ -483,7 +494,8 @@ impl<'a> Run<'a> {
             self.ctx.static_rules.clone(),
             self.ctx.extra_rules.clone(),
             call_cancel.clone(),
-        );
+        )
+        .with_parent_rules(self.ctx.parent_rules.clone());
         let tool_ctx = ToolCtx {
             session_id: self.ctx.session_id.clone(),
             message_id: self.message_id(),
@@ -493,6 +505,7 @@ impl<'a> Run<'a> {
             cancel: call_cancel.clone(),
             ask,
             metadata: metadata_sink,
+            spawner: self.ctx.spawner.clone(),
         };
 
         let result = tokio::select! {