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-app/Cargo.toml

@@ -12,12 +12,14 @@ harness-mcp = { workspace = true }
 harness-lsp = { workspace = true }
 tokio = { workspace = true }
 tokio-util = { workspace = true }
+async-trait = { workspace = true }
 dirs = { workspace = true }
 thiserror = { workspace = true }
 tracing = { workspace = true }
 
 [dev-dependencies]
 tempfile = { workspace = true }
+futures = { workspace = true }
 
 [lints]
 workspace = true