feat: add secrets management via system keyring

SecretsManager using keyring crate (linux-native/libsecret). Store/get/
delete/list with __bterminal_keys__ metadata tracking. SettingsTab
Secrets section. No plaintext fallback.

v2/src/lib/adapters/secrets-bridge.ts

@@ -0,0 +1,39 @@
+import { invoke } from '@tauri-apps/api/core';
+
+/** Store a secret in the system keyring. */
+export async function storeSecret(key: string, value: string): Promise<void> {
+  return invoke('secrets_store', { key, value });
+}
+
+/** Retrieve a secret from the system keyring. Returns null if not found. */
+export async function getSecret(key: string): Promise<string | null> {
+  return invoke('secrets_get', { key });
+}
+
+/** Delete a secret from the system keyring. */
+export async function deleteSecret(key: string): Promise<void> {
+  return invoke('secrets_delete', { key });
+}
+
+/** List keys that have been stored in the keyring. */
+export async function listSecrets(): Promise<string[]> {
+  return invoke('secrets_list');
+}
+
+/** Check if the system keyring is available. */
+export async function hasKeyring(): Promise<boolean> {
+  return invoke('secrets_has_keyring');
+}
+
+/** Get the list of known/recognized secret key identifiers. */
+export async function knownSecretKeys(): Promise<string[]> {
+  return invoke('secrets_known_keys');
+}
+
+/** Human-readable labels for known secret keys. */
+export const SECRET_KEY_LABELS: Record<string, string> = {
+  anthropic_api_key: 'Anthropic API Key',
+  openai_api_key: 'OpenAI API Key',
+  github_token: 'GitHub Token',
+  relay_token: 'Relay Token',
+};