Integrate the remote protected-operation broker on current main

Outcome

Mecatl can use a separately deployed protected-operation broker for the first external-integration proof, while honestly surfacing that this initial broker is single-replica and restart-interrupting.

Proposed approach

Port the existing remote mecabroker baseline onto an explicit current-main integration branch rather than merging its divergent branch wholesale. Preserve its authenticated workload boundary, secret-free protocol, bounded callback handling, attachment/incarnation behavior, and safe failure semantics.

The broker is the authority and credential-custody service. Mecatl must not receive provider refresh credentials or provider access tokens. A future local sidecar, if wanted, is only a narrow client proxy and does not become a second authority or credential store.

Note: this is unrelated to the existing in-repo mecak8s "MCP broker" (internal/adapter/mcpbroker, e.g. #1352, #1367, #1375, #1379). That subsystem brokers MCP tool connectivity within a running mecak8s deployment. This issue is about a separately deployed, external protected-operation/credential-custody service.

Not in this issue

Broker replication, transparent continuity during restart, a generic proxy API, or a final sidecar-versus-direct-client choice.

Proof

• An expired attachment handle does not let a different workload inherit a retained logical session.
• A recognized broker incarnation replacement safely rebinds pre-prompt enrollment; generic state loss fails closed.
• A callback timeout cannot reach the callback handler late.
• A broker restart visibly interrupts protected work rather than silently claiming continuity.

Start by locating the existing remote mecabroker baseline and comparing it with current main; the issue names no files or tests. Use the proposed approach and proof cases as the acceptance guide: preserve the authenticated, secret-free boundary, safe failure behavior, attachment and incarnation handling, and visible interruption on broker restart.