feat: add hidden CODER_AGENT_IS_SUB_AGENT flag to coder agent (#17783)

Closes https://github.com/coder/internal/issues/620

Adds a new, hidden, flag `CODER_AGENT_IS_SUB_AGENT` to the `coder agent`
command.

agent/agent.go

@@ -89,6 +89,7 @@ type Options struct {
 	ServiceBannerRefreshInterval time.Duration
 	BlockFileTransfer            bool
 	Execer                       agentexec.Execer
+	SubAgent                     bool
 
 	ExperimentalDevcontainersEnabled bool
 	ContainerAPIOptions              []agentcontainers.Option // Enable ExperimentalDevcontainersEnabled for these to be effective.
@@ -190,6 +191,8 @@ func New(options Options) Agent {
 		metrics:            newAgentMetrics(prometheusRegistry),
 		execer:             options.Execer,
 
+		subAgent: options.SubAgent,
+
 		experimentalDevcontainersEnabled: options.ExperimentalDevcontainersEnabled,
 		containerAPIOptions:              options.ContainerAPIOptions,
 	}
@@ -272,6 +275,8 @@ type agent struct {
 	metrics *agentMetrics
 	execer  agentexec.Execer
 
+	subAgent bool
+
 	experimentalDevcontainersEnabled bool
 	containerAPIOptions              []agentcontainers.Option
 	containerAPI                     atomic.Pointer[agentcontainers.API] // Set by apiHandler.