docs: surface security features and test coverage in README

[akuehner]

Summary

• Add Security section after Architecture documenting five code-level properties: 30-day token TTL with upgrade migration, atomic rename-after-write with mode 0o600, append-only audit log, progressive lockout with per-IP and per-username tracking, and OS-level isolation in multi-user mode
• Add Postinstall / systemd subsection under Install explaining the Linux systemd unit registration and how to skip with --ignore-scripts
• Add Testing section listing the 12 test surface areas covered by the 26-file test suite

Documentation-only change. No code modified.

Test plan

• npm test — 303 tests, 0 failures
• All claims verified against source: lib/server-auth.js (TOKEN_TTL_MS, lockout), lib/store.js (atomic write, 0o600), lib/audit.js (append-only log), lib/os-users.js (OS isolation), scripts/postinstall.js (systemd), test/ (26 test files)

[clagentic-release-bot]

This issue has been resolved in version 1.5.0-beta.2 (beta).

To update, run:

npx @clagentic/console@1.5.0-beta.2