feat(agent): stream live agent output to issue detail page

When an agent is working on an issue, users can now see real-time output
in the issue detail page instead of waiting for completion.

Backend:
- Add task_message table and migration for persisting agent messages
- Add POST /api/daemon/tasks/{id}/messages endpoint for daemon to report
  structured messages (tool_use, tool_result, text, error) in batches
- Add GET /api/daemon/tasks/{id}/messages for catch-up after reconnect
- Add GET /api/issues/{id}/active-task to check for running tasks
- Broadcast task:message events via WebSocket
- Daemon forwards agent session messages with 500ms text throttling

Frontend:
- Add AgentLiveCard component showing live tool calls, text output,
  and progress indicators with auto-scroll
- Wire into issue detail timeline with WS subscription and HTTP catch-up
- Card appears when agent is working, disappears on completion/failure

server/pkg/protocol/events.go

@@ -22,6 +22,7 @@ const (
 	EventTaskProgress  = "task:progress"
 	EventTaskCompleted = "task:completed"
 	EventTaskFailed    = "task:failed"
+	EventTaskMessage   = "task:message"
 
 	// Inbox events
 	EventInboxNew           = "inbox:new"

server/pkg/protocol/messages.go

@@ -31,6 +31,18 @@ type TaskCompletedPayload struct {
 	Output string `json:"output,omitempty"`
 }
 
+// TaskMessagePayload represents a single agent execution message (tool call, text, etc.)
+type TaskMessagePayload struct {
+	TaskID  string         `json:"task_id"`
+	IssueID string         `json:"issue_id,omitempty"`
+	Seq     int            `json:"seq"`
+	Type    string         `json:"type"`              // "text", "tool_use", "tool_result", "error"
+	Tool    string         `json:"tool,omitempty"`     // tool name for tool_use/tool_result
+	Content string         `json:"content,omitempty"`  // text content
+	Input   map[string]any `json:"input,omitempty"`    // tool input (tool_use only)
+	Output  string         `json:"output,omitempty"`   // tool output (tool_result only)
+}
+
 // DaemonRegisterPayload is sent from daemon to server on connection.
 type DaemonRegisterPayload struct {
 	DaemonID string        `json:"daemon_id"`