M4: subagent context-file reporting to the job board

Lets subagents report context files back to the job board (surfaced from the read tool) so the parent session's next-turn context includes what a background child has been looking at.

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

@@ -411,6 +411,7 @@ mod tests {
             now: 1,
             spawner: None,
             job_board: None,
+            context_reporter: None,
         }
     }
 

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

@@ -11,7 +11,8 @@ use crate::llm::{FinishReason, LlmEvent, LlmEventStream, ProviderError};
 use crate::permission::{PermissionService, Ruleset};
 use crate::store::Store;
 use crate::tool::{
-    MetadataSink, PermissionHandle, SubagentSpawner, Tool, ToolCtx, ToolError, ToolRegistry,
+    ContextReporter, MetadataSink, PermissionHandle, SubagentSpawner, Tool, ToolCtx, ToolError,
+    ToolRegistry,
 };
 use crate::types::{Message, MessageId, Part, PartBody, PartId, SessionId, TokenUsage, ToolState};
 
@@ -65,6 +66,9 @@ pub struct StepContext {
     /// 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>>,
+    /// Present in subagent sessions: reports read files to this session's job on the parent
+    /// board. `None` for root sessions (nothing to report to).
+    pub context_reporter: Option<Arc<dyn ContextReporter>>,
 }
 
 struct FlushTracker {
@@ -506,6 +510,7 @@ impl<'a> Run<'a> {
             ask,
             metadata: metadata_sink,
             spawner: self.ctx.spawner.clone(),
+            context_reporter: self.ctx.context_reporter.clone(),
         };
 
         let result = tokio::select! {

crates/harness-core/src/tool/mod.rs

@@ -54,6 +54,14 @@ pub trait SubagentSpawner: Send + Sync {
     async fn spawn(&self, req: SpawnRequest) -> Result<SpawnOutcome, SpawnError>;
 }
 
+/// Lets a child session report the files it read to its job board entry, so a completed
+/// specialist advertises what it already looked at (docs/04-multiagent.md). Present only in
+/// subagent sessions; the spawner wires it to the right board + job.
+#[async_trait]
+pub trait ContextReporter: Send + Sync {
+    async fn report_file(&self, path: String, lines: u32);
+}
+
 #[derive(Debug, thiserror::Error)]
 pub enum ToolError {
     #[error("permission denied")]
@@ -185,6 +193,8 @@ pub struct ToolCtx {
     /// Present when the engine can spawn subagents (the `task` tool's capability). `None`
     /// in headless/test contexts with no orchestration wired in.
     pub spawner: Option<Arc<dyn SubagentSpawner>>,
+    /// Present in subagent sessions: lets the read tool report files to the job board.
+    pub context_reporter: Option<Arc<dyn ContextReporter>>,
 }
 
 #[derive(Debug)]