fix: pass agent context config explicitly instead of reading env (#24759)

The CODER_AGENT_EXP_* env vars are agent-internal options. When set
in the workspace environment they leak to MCP subprocesses and user
shells.

ReadEnvConfig() captures the values and ClearEnvVars() strips them
before the reinit loop, so config survives agent restarts. NewAPI
and ReadEnvConfig both use applyDefaults() to fill zero fields.
The chatd test passes config via agenttest.WithContextConfigFromEnv().

coderd/x/chatd/chatd_test.go

@@ -8324,7 +8324,7 @@ func TestAgentContextFilesAndSkillsLoadedIntoChat(t *testing.T) {
 	workspace := coderdtest.CreateWorkspace(t, client, template.ID)
 	coderdtest.AwaitWorkspaceBuildJobCompleted(t, client, workspace.LatestBuild.ID)
 
-	_ = agenttest.New(t, client.URL, agentToken)
+	_ = agenttest.New(t, client.URL, agentToken, agenttest.WithContextConfigFromEnv())
 	coderdtest.NewWorkspaceAgentWaiter(t, client, workspace.ID).Wait()
 
 	// Capture LLM requests so we can inspect the system prompt.