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>

Cargo.lock

@@ -677,6 +677,7 @@ dependencies = [
  "harness-mcp",
  "harness-providers",
  "harness-tools",
+ "serde_json",
  "tempfile",
  "thiserror 2.0.18",
  "tokio",