feat(httpapi): Prometheus /metrics endpoint

[prashar32]

Adds an authenticated Prometheus /metrics endpoint so platform teams can watch RiskKernel's own state alongside everything else they run.

What it exposes

GET /metrics returns the Prometheus text exposition format (version 0.0.4), sourced from the durable store:

• riskkernel_runs_total{status} — governed runs by lifecycle status (running / halted / cancelled)
• riskkernel_runs_halted_total{reason} — halted runs by halt reason (token/dollar/loop/time budget, cancelled)
• riskkernel_spend_dollars_total — total spend in dollars, summed from the cost ledger
• riskkernel_tokens_total — total prompt + completion tokens
• riskkernel_model_calls_total — priced model calls recorded in the ledger
• riskkernel_approvals_pending — pending human-in-the-loop approval-queue depth

Each metric carries proper # HELP and # TYPE lines; counters for monotonic spend/tokens/calls, gauges for the live status/queue counts.

Notes

• No new dependency. The exposition format is hand-rolled — the surface is tiny and a minimal dependency graph is the rule. No prometheus/client_golang.
• Honors the no-telemetry posture. This is local metrics the user scrapes: numbers are derived on the fly from the SQLite state the user already owns, nothing is emitted anywhere, and no prompt content or PII is exposed.
• Authenticated like the rest of the API (bearer token when one is configured), and registered only when a durable store is the source of truth.
• Output is deterministic (labels and series sorted), and a fresh daemon still emits a zero riskkernel_runs_total series so a scrape never reads an empty metric as a failed scrape.
• docs/METRICS.md documents the metric list and includes an example Prometheus scrape config.

Tests

New internal/httpapi/metrics_test.go (mirrors the newTestServer harness): seeds a running / halted / cancelled run plus a pending approval, scrapes /metrics, and asserts 200, the text/plain; version=0.0.4 Content-Type, and the expected metric lines and values. Also covers the empty-daemon case and that auth is enforced (401 without the token, 200 with it).

gofmt -l is clean, go vet ./... passes, and go test ./... passes.

Closes #91.