[功能建议] 支持 per-agent / per-namespace trust registry

[nnn228085-star]

问题描述：
当前 trust registry 位于 ~/.agentguard/data/registry.json，隐式在机器上所有 agent 之间共享。在有严格隔离要求的多 agent 部署中，这与 per-agent 隔离原则冲突。需要支持 per-agent 或 per-namespace 的独立 trust registry。

[morgan-coded]

Hi, I'm looking at the per-agent/per-namespace trust registry request. Before proposing code, should the registry scope be selected by an explicit config, CLI, or env namespace value, or inferred from the active agent integration?

I'm asking because ~/.agentguard/data/registry.json is machine-wide today, and the storage key shape affects migration and backward compatibility. If maintainers want a PR here, I'd start with one scoped namespace selector plus a migration-safe fallback to the current registry path, rather than changing all trust-record behavior at once.

[morgan-coded]

Mapped the migration for this at head d18e142: the registry format and every existing config key survive unchanged — the only changed surface is the path derivation in src/registry/storage.ts:31-38 plus one additive config key.

Per-path registries are reachable today (--registry-path on the MCP server and skill CLIs, createAgentGuard({registryPath}), AGENTGUARD_HOME), so this reduces to a first-class selector, not new capability. One correction: the live default is ~/.agentguard/registry.json — the data/ path in the issue body is the in-repo seed, not the user-facing file.

Any of config, env, or agentHost inference lands on the same shape. If you pick one I can turn this into a scoped PR; if not, no harm leaving it parked.