feat(executor): introduce AgentChannel seam for runner signals

Defines AgentChannel — the normalized interface by which a runner reports
agent-originated signals (AskUser, ReportSummary, SpawnSubtask,
RecordProgress) — plus a default storeChannel implementation backed by
storage. Runner.Run now takes an AgentChannel; the pool constructs one
per execution.

The file transport routes its post-exit summary detection through
ch.ReportSummary (buffered onto the execution so the pool still applies
its extract/synthesize fallbacks, no double-write). AskUser returns
ErrAgentBlocked since write-and-exit cannot answer in-session; question
persistence stays with the pool's BlockedError handling. SpawnSubtask
and RecordProgress are implemented and tested, ready for the MCP
transport in Phase 2 where the channel becomes fully load-bearing.

Store gains CreateEvent so the channel can emit agent_message events.

https://claude.ai/code/session_01SESwn7kQ7oP62trWw6pc39

1 files changed, 3 insertions, 3 deletions

diff --git a/internal/executor/local_test.go b/internal/executor/local_test.go
index d8ab678..8aa8791 100644
--- a/internal/executor/local_test.go
+++ b/internal/executor/local_test.go
@@ -72,7 +72,7 @@ func TestLocalRunner_Run_WritesStreamJSON(t *testing.T) {
 	}
 	exec := &storage.Execution{ID: uuid.New().String(), TaskID: tt.ID}
 
-	if err := r.Run(context.Background(), tt, exec); err != nil {
+	if err := r.Run(context.Background(), tt, exec, newStoreChannel(nil, tt.ID, exec)); err != nil {
 		t.Fatalf("Run: %v", err)
 	}
 
@@ -121,7 +121,7 @@ func TestLocalRunner_Run_NoClient_Errors(t *testing.T) {
 	r := &LocalRunner{LogDir: t.TempDir()}
 	tt := &task.Task{ID: "x", Agent: task.AgentConfig{Instructions: "hi"}}
 	exec := &storage.Execution{ID: "exec-x"}
-	err := r.Run(context.Background(), tt, exec)
+	err := r.Run(context.Background(), tt, exec, newStoreChannel(nil, tt.ID, exec))
 	if err == nil || !strings.Contains(err.Error(), "no LLM client") {
 		t.Errorf("expected 'no LLM client' error, got %v", err)
 	}
@@ -134,7 +134,7 @@ func TestLocalRunner_Run_EmptyInstructions_Errors(t *testing.T) {
 	}
 	tt := &task.Task{ID: "x", Agent: task.AgentConfig{}}
 	exec := &storage.Execution{ID: "exec-x"}
-	err := r.Run(context.Background(), tt, exec)
+	err := r.Run(context.Background(), tt, exec, newStoreChannel(nil, tt.ID, exec))
 	if err == nil || !strings.Contains(err.Error(), "empty instructions") {
 		t.Errorf("expected empty-instructions error, got %v", err)
 	}