Client API: Prometheus HTTP SD endpoint for local allocations

### Proposal

Add `GET /v1/client/allocations/prometheus-sd` to the client agent HTTP API, serving the node's running allocations as [Prometheus HTTP SD](https://prometheus.io/docs/prometheus/latest/http_sd/) target groups.

One target group per allocated port of every running allocation, labeled with `__meta_nomad_*` labels (namespace, job, task group, allocation, node, port) plus job/group meta as `__meta_nomad_meta_`. A `?port=` query parameter filters to a single port label (e.g. `?port=metrics`). The endpoint would serve local client state only and require `node:read`.

Implementation sketch: ~200 lines in `command/agent/` plus a client method. Test plan: unit tests covering target-group rendering, port filtering, ACL enforcement, and IPv6 host-IP bracketing.

### Use-cases

Scraping per-allocation metrics with a node-local collector (e.g. a Prometheus agent per node). Each node's collector discovers its own allocations directly from the local client, so scrape-target discovery fans out to the clients instead of funneling through the servers — no server round-trip, no single discovery bottleneck, and discovery keeps working on a node even when servers are briefly unreachable.

Start in command/agent/ and trace the client HTTP API handlers and client method used for local allocation state. Add the Prometheus HTTP SD target-group response with port filtering, Nomad labels, ACL enforcement, and IPv6 host-IP handling, then cover rendering, filtering, ACL, and IPv6 behavior with unit tests.

Written by the indexing model from the issue text.