feat(prometheus): honor PROMETHEUS_URL as the default server URL

[jedi-tal]

Problem

PROMETHEUS_URL is documented in the README as "Default Prometheus server URL", but nothing reads it. Every prometheus_* handler hard-codes the fallback:

prometheusURL := mcp.ParseString(request, "prometheus_url", "http://localhost:9090")

and the chart's tools.prometheus.url value is never passed to the container (#63, #36).

So the only way to reach a Prometheus that is not on localhost is for the model to pass prometheus_url on every single call. Since the parameter is optional and the schema advertises a plausible default, models routinely omit it — the call fails with connection refused against a port nothing serves inside the tool server's pod, and the agent burns a turn retrying.

That is not theoretical: on a scheduled cluster-health agent of ours (tool server pointed at an in-cluster Thanos Query), one run omitted the parameter on its first 12 calls and spent its entire 24-call budget re-issuing them, shipping a report with several findings left unidentified.

Change

• defaultPrometheusURL() reads PROMETHEUS_URL (trimmed), falling back to http://localhost:9090 when unset — existing deployments are unchanged.
• The prometheus_url parameter description is built from that same value, so the tool schema advertises the default a caller will actually get rather than always claiming localhost. This is the part that stops the model from omitting it wrongly.
• The chart renders PROMETHEUS_URL from tools.prometheus.url, only when set. That value's previous default was schemeless (prometheus.kagent.svc.cluster.local:9090) and would be rejected by security.ValidateURL ("must start with http:// or https://"), so it now defaults to empty = tool default.

An explicit prometheus_url argument still wins when supplied.

Test

• TestDefaultPrometheusURL — unset → localhost fallback; set (with surrounding whitespace) → honored, and reflected in the parameter description.
• TestPrometheusURLDefaultsToEnv — a call that omits prometheus_url reaches the env-configured server, asserted on the recorded request URL.

go vet + go test ./pkg/prometheus/... pass; gofmt clean. Chart rendered both ways: env present when tools.prometheus.url is set, absent when it is not.

Closes #63