CM-68944: Fetch AI guardrail config from the platform - #541

…cally

Enforcement config (per-agent Off/Report/Block modes and sensitive-path globs)
is now platform-owned. session-start fetches the tenant's resolved config into
~/.cycode/ai-guardrails-config.json (TTL- and tenant-gated); scans only read the
cache and overlay it onto the local policy, so the hot path never touches the
network. No cache means built-in Report defaults, always synchronous.

- An event whose guardrails are all Off exits before any scan, event or auth.
- The two FileRead guardrails (content scan, sensitive path) keep independent
  modes via a per-check action.
- ai-guardrails.yaml is reduced to operational knobs (timeout, max bytes,
  fail_open); enforcement keys older CLIs wrote are stripped, and --mode on
  install is deprecated.
- Atomic write and corrupt-file quarantine are shared helpers in path_utils.

Co-Authored-By: Claude Code <noreply@anthropic.com>

Review follow-up on the guardrail config cache.

DEFAULT_POLICY carried prompt/file_read/mcp enforcement keys that the platform
overlay overwrites on every scan, so they were dead defaults the installer still
wrote into the user's ai-guardrails.yaml. It now holds knobs only, with the
sensitive-path fallback list moved to DEFAULT_SENSITIVE_PATH_GLOBS.

load_policy whitelists the keys a local file may contribute instead of stripping
the ones it may not. The strip-list had already missed mcp.scan_arguments, which
let a repo-level file switch off MCP argument scanning; a whitelist cannot miss a
key. strip_platform_managed_keys stays for the write path only, where it drops
platform sections while keeping whatever else the user put in the file.

The handlers' `enabled` gates and the mcp scan_arguments gate go with them:
nothing could set them any more, and an all-Off event is skipped in scan_command.

The matrix cell vocabulary becomes GuardrailCellMode, kept apart from
GuardrailsMode so Off does not leak into `ai-guardrails install --mode`.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>