docs(go): audit README and AGENTS

Align the README API reference with the exported surface, add the
upgrade note, document the shared modules and Linux test setup in
AGENTS.md, and scope the Grok expandable-path assertion to Windows.

Claude-Session: https://claude.ai/code/session_01Agoc9bRVE9KL7mmWtey9Vq

cmd/elydora/plugins/grok_test.go

@@ -36,7 +36,7 @@ func TestGrokCommandRoundTripAndRejectsInjection(t *testing.T) {
 		!sameManagedPath(script, fixture.guardPath) {
 		t.Fatalf("parsed Grok command = %q, %q, %v", executable, script, ok)
 	}
-	if strings.Contains(command, "%ELYDORA_HOOK_PATH%") {
+	if runtime.GOOS == "windows" && strings.Contains(command, "%ELYDORA_HOOK_PATH%") {
 		t.Fatalf("Grok command exposes an expandable path: %q", command)
 	}
 	if _, err := buildEncodedCommand("Grok", "node", fixture.guardPath); err == nil {