fix(security): tighten on-disk credential file modes (700/600) (#87)

Credential-bearing OCP artifacts were created at default umask (0644/0755),
making them world-readable on multi-user hosts. This commit hardens them at
writer time and adds an idempotent startup reconciliation so existing prod
installs are fixed automatically on next service restart.

Changes:
- keys.mjs: mkdirSync(OCP_DIR, { mode: 0o700 }) + chmodSync after to handle
  pre-existing dirs; chmodSync(DB_PATH, 0o600) after first getDb() open.
- setup.mjs: chmodSync(plistPath, 0o600) after writeFileSync on macOS;
  chmodSync(unitPath, 0o600) after writeFileSync on Linux.
- server.mjs: _tightenFileModesIfPossible() reconciliation block — idempotently
  chmods ~/.ocp (700), ~/.ocp/admin-key (600), ~/.ocp/ocp.db (600) on startup;
  emits a single info-level log line when any file is tightened; ignores ENOENT
  and wraps EPERM in a warn log so startup is never crashed by chmod failure.

Backward compat: all files remain accessible to the same-user owner; 0o600
is still fully readable/writable by the process. Existing prod boxes with
old 0644 ~/.ocp directories get fixed-up on next launchd/systemd restart
without any manual intervention.

cli.js citation: this change is OCP-internal file-permission hardening only.
No cli.js function corresponds to chmod or credential-file management. This
is an OCP-local security improvement that is out of scope for the cli.js
citation requirement per ALIGNMENT.md Rule 2; the cli.js boundary applies
to proxy protocol and API surface changes, not host-filesystem hardening.
Identified during privacy/security audit — .claude/research/ocp-security-audit.md.

Co-authored-by: dtzp555 <dtzp555@gmail.com>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

keys.mjs

@@ -3,11 +3,13 @@
 import { DatabaseSync } from "node:sqlite";
 import { randomBytes, createHash } from "node:crypto";
 import { join } from "node:path";
-import { mkdirSync } from "node:fs";
+import { mkdirSync, chmodSync } from "node:fs";
 import { homedir } from "node:os";
 
 const OCP_DIR = join(homedir(), ".ocp");
-mkdirSync(OCP_DIR, { recursive: true });
+mkdirSync(OCP_DIR, { recursive: true, mode: 0o700 });
+// Tighten the directory mode in case it already existed with broader permissions.
+try { chmodSync(OCP_DIR, 0o700); } catch { /* ignore EPERM on pre-existing dirs */ }
 const DB_PATH = join(OCP_DIR, "ocp.db");
 
 let db;
@@ -18,6 +20,8 @@ export function getDb() {
     db.exec("PRAGMA journal_mode = WAL");
     db.exec("PRAGMA foreign_keys = ON");
     initSchema();
+    // Tighten mode on the DB file (0600) after creation / first open.
+    try { chmodSync(DB_PATH, 0o600); } catch { /* ignore — same-user access still works */ }
   }
   return db;
 }