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, 4 insertions, 4 deletions

diff --git a/internal/executor/claude_test.go b/internal/executor/claude_test.go
index c01e160..0e76260 100644
--- a/internal/executor/claude_test.go
+++ b/internal/executor/claude_test.go
@@ -259,7 +259,7 @@ func TestClaudeRunner_Run_ResumeSetsSessionIDFromResumeSession(t *testing.T) {
 	}
 
 	// Run completes successfully (binary is "true").
-	_ = r.Run(context.Background(), tk, exec)
+	_ = r.Run(context.Background(), tk, exec, newStoreChannel(nil, tk.ID, exec))
 
 	// SessionID must be the original session (ResumeSessionID), not the new
 	// exec's own ID. If it were exec.ID, a second blocked-then-resumed cycle
@@ -284,7 +284,7 @@ func TestClaudeRunner_Run_InaccessibleWorkingDir_ReturnsError(t *testing.T) {
 	}
 	exec := &storage.Execution{ID: "test-exec"}
 
-	err := r.Run(context.Background(), tk, exec)
+	err := r.Run(context.Background(), tk, exec, newStoreChannel(nil, tk.ID, exec))
 
 	if err == nil {
 		t.Fatal("expected error for inaccessible working_dir, got nil")
@@ -732,7 +732,7 @@ func TestClaudeRunner_Run_ResumeUsesStoredSandboxDir(t *testing.T) {
 		SandboxDir:      sandboxDir,
 	}
 
-	_ = r.Run(context.Background(), tk, exec)
+	_ = r.Run(context.Background(), tk, exec, newStoreChannel(nil, tk.ID, exec))
 
 	got, err := os.ReadFile(cwdFile)
 	if err != nil {
@@ -778,7 +778,7 @@ func TestClaudeRunner_Run_StaleSandboxDir_ClonesAfresh(t *testing.T) {
 		SandboxDir:      staleSandbox,
 	}
 
-	if err := r.Run(context.Background(), tk, e); err != nil {
+	if err := r.Run(context.Background(), tk, e, newStoreChannel(nil, tk.ID, e)); err != nil {
 		t.Fatalf("Run with stale sandbox: %v", err)
 	}