harness-tools + app: subagent context-file reporting to the job board (M4)

A ContextReporter wired into subagent sessions lets the read tool advertise the
files it reads (≥10 lines) on its job board entry, so a completed specialist
shows what it already looked at. Root sessions report nothing (no parent board).

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

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! {