util/syspolicy: finish adding ts_omit_syspolicy build tags, tests

Fixes #16998
Updates #12614

Change-Id: Idf2b1657898111df4be31f356091b2376d0d7f0b
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

client/local/syspolicy.go

@@ -0,0 +1,40 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !ts_omit_syspolicy
+
+package local
+
+import (
+	"context"
+	"net/http"
+
+	"tailscale.com/util/syspolicy/setting"
+)
+
+// GetEffectivePolicy returns the effective policy for the specified scope.
+func (lc *Client) GetEffectivePolicy(ctx context.Context, scope setting.PolicyScope) (*setting.Snapshot, error) {
+	scopeID, err := scope.MarshalText()
+	if err != nil {
+		return nil, err
+	}
+	body, err := lc.get200(ctx, "/localapi/v0/policy/"+string(scopeID))
+	if err != nil {
+		return nil, err
+	}
+	return decodeJSON[*setting.Snapshot](body)
+}
+
+// ReloadEffectivePolicy reloads the effective policy for the specified scope
+// by reading and merging policy settings from all applicable policy sources.
+func (lc *Client) ReloadEffectivePolicy(ctx context.Context, scope setting.PolicyScope) (*setting.Snapshot, error) {
+	scopeID, err := scope.MarshalText()
+	if err != nil {
+		return nil, err
+	}
+	body, err := lc.send(ctx, "POST", "/localapi/v0/policy/"+string(scopeID), 200, http.NoBody)
+	if err != nil {
+		return nil, err
+	}
+	return decodeJSON[*setting.Snapshot](body)
+}