feat(server): `_meta.vouch_trust` block on every response

[plind-junior]

What you're trying to do

Once #6 lands, every response should surface the trust state the call was treated as — so clients can detect when they're being confined and surface that in their own UI. gbrain's _meta.brain_hot_memory shows the same shape: server-attached, opt-in to render, never authoritative.

Suggested shape

• Every kb.* response carries _meta.vouch_trust: {remote: bool, caller_kind: str, auth_subject: str | null}.
• The block is read-only metadata; client mutations are ignored.
• Documented under the JSONL request/response shape section of README.

Acceptance

• An HTTP-MCP call surfaces remote: true, caller_kind: "mcp_http"; a CLI call surfaces remote: false, caller_kind: "cli".
• Test asserts the field appears on every read response.

Out of scope

• Cryptographic signing of the trust block.