fix(handler): attribute agent CLI actions to agent identity

When agents use the multica CLI during task execution, their comments,
issue updates, and issue creations were attributed to the daemon's user
(via JWT) instead of the agent. Pass MULTICA_AGENT_ID env var from the
daemon, send X-Agent-ID header from the CLI client, and use it in
handlers to set the correct author/actor identity.

server/internal/cli/client.go

@@ -21,6 +21,7 @@ type APIClient struct {
 	BaseURL     string
 	WorkspaceID string
 	Token       string
+	AgentID     string // When set, requests are attributed to this agent instead of the user.
 	HTTPClient  *http.Client
 }
 
@@ -41,6 +42,9 @@ func (c *APIClient) setHeaders(req *http.Request) {
 	if c.WorkspaceID != "" {
 		req.Header.Set("X-Workspace-ID", c.WorkspaceID)
 	}
+	if c.AgentID != "" {
+		req.Header.Set("X-Agent-ID", c.AgentID)
+	}
 }
 
 // GetJSON performs a GET request and decodes the JSON response.