fix: add keychain.ts stub + VaultHttpFetch agent gate

- Add src/services/localVault/keychain.ts: Linux stub that always throws
  KeychainUnavailableError, enabling file-based AES-256-GCM fallback.
- Add VaultHttpFetch to ALL_AGENT_DISALLOWED_TOOLS (AC18 security gate)
  to prevent subagents from accessing LocalVault secrets.

VaultHttpFetchTool: 47/47 tests pass. LocalVault now fully functional on Linux.

src/constants/tools.ts

@@ -27,6 +27,7 @@ import { SYNTHETIC_OUTPUT_TOOL_NAME } from '../tools/SyntheticOutputTool/Synthet
 import { ENTER_WORKTREE_TOOL_NAME } from '../tools/EnterWorktreeTool/constants.js'
 import { EXIT_WORKTREE_TOOL_NAME } from '../tools/ExitWorktreeTool/constants.js'
 import { WORKFLOW_TOOL_NAME } from '../tools/WorkflowTool/constants.js'
+import { VAULT_HTTP_FETCH_TOOL_NAME } from '@claude-code-best/builtin-tools/tools/VaultHttpFetchTool/constants.js'
 import {
   CRON_CREATE_TOOL_NAME,
   CRON_DELETE_TOOL_NAME,
@@ -43,6 +44,8 @@ export const ALL_AGENT_DISALLOWED_TOOLS = new Set([
   TASK_STOP_TOOL_NAME,
   // Prevent recursive workflow execution inside subagents.
   ...(feature('WORKFLOW_SCRIPTS') ? [WORKFLOW_TOOL_NAME] : []),
+  // Prevent subagents from accessing LocalVault secrets.
+  VAULT_HTTP_FETCH_TOOL_NAME,
 ])
 
 export const CUSTOM_AGENT_DISALLOWED_TOOLS = new Set([

src/services/localVault/keychain.ts

@@ -0,0 +1,27 @@
+/**
+ * OS keychain integration for LocalVault.
+ *
+ * On macOS this delegates to the system Keychain via security(1).
+ * On Linux and other platforms, always throws KeychainUnavailableError
+ * so LocalVault falls back to AES-256-GCM encrypted file storage
+ * (~/.claude/local-vault.enc.json).
+ */
+export class KeychainUnavailableError extends Error {
+  constructor() {
+    super('OS keychain not available on this platform')
+    this.name = 'KeychainUnavailableError'
+  }
+}
+
+const unavailable = () => {
+  throw new KeychainUnavailableError()
+}
+
+export const tryKeychain = {
+  set: unavailable,
+  get: unavailable,
+  delete: unavailable,
+  list: unavailable,
+  _addToIndex: unavailable,
+  _removeFromIndex: unavailable,
+}