Add ManifestStore that loads TOML agent type manifests from a directory.
Each manifest defines allowed tools, path allowlists, network egress
policies, lineage constraints (can_spawn), and max spawn depth.
Includes validation, reload support, and lookup by ID or name.
14 manifest tests + 8 existing = 22 total, clippy clean.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
