Replace noopMessenger stub with live AgentMessenger in daemon wiring #79

Context

PR #65 wires a stub noopMessenger into the Session Manager:

// backend/internal/daemon/lifecycle_wiring.go:35-39
type noopMessenger struct{}

func (noopMessenger) Send(context.Context, domain.SessionID, string) error { return nil }

It's passed as Messenger: noopMessenger{} at lifecycle_wiring.go:60 and reaches the SM at session_manager/manager.go (m.messenger.Send in Manager.Send). The contract lives at backend/internal/ports/outbound.go (AgentMessenger.Send(ctx, SessionID, string) error).

While the stub is in place, Manager.Send silently no-ops — any caller (REST /sessions/{id}/messages, controllers, future automation) returns success without anything actually reaching the running agent.

PR #74 (feat/aa-43-ao-send-pane-ping) implements the live messenger (zellij pane ping + ao send CLI). This issue tracks swapping it in.

Scope

• Once PR feat(messenger): ao send CLI + live zellij pane ping (aa-43) #74 lands, replace noopMessenger{} at lifecycle_wiring.go:60 with the real ports.AgentMessenger implementation from PR feat(messenger): ao send CLI + live zellij pane ping (aa-43) #74.
• Remove the noopMessenger type from lifecycle_wiring.go:35-39.
• Add a wiring_test.go assertion that startSession wires a non-noop messenger (e.g. resolves to the concrete type, or behaves as expected on Send).
• Confirm a Manager.Send call against a live worker actually reaches the agent pane in an end-to-end test or smoke note.

Blocked-by / blocks

• Blocked by: PR Add agent adapters and wire per-session agents into the session manager #65 (the noop is introduced there) and PR feat(messenger): ao send CLI + live zellij pane ping (aa-43) #74 (the live implementation).
• Blocks: any feature relying on injected agent nudges (e.g. orchestrator → worker prompts, reviewer-driven retries).

Risk if left unaddressed

Silent failure mode: callers of /sessions/{id}/messages get HTTP 200 with no delivery. Easy to miss in manual testing. Worth landing alongside PR #74 rather than as a later cleanup.