Add AGENTS.md with observability guidance

AGENTS.md

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+# Agent Guidelines
+
+Swarf is the UCAN revocation service; `README.md` describes running it and its CLI.
+
+## Observability
+
+When a change adds or alters behavior someone depends on, decide how on-call
+would see it working and see it fail, and add that in the same PR where it is
+useful: logs, and an alert or dashboard panel in fil-forge/infra-central. Skip
+it for changes nothing operational depends on. When a feature's health is best
+read as a number, say so in the PR description, naming the metric and
+[FIL-1152](https://linear.app/filecoin-foundation/issue/FIL-1152). "Adding a
+signal" in `docs/observability.md` says how to do each.

docs/observability.md

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+# Observability
+
+How this service's failures reach on-call, and what a change adds so that they
+do.
+
+## Adding a signal
+
+- **Logs are what reaches Grafana today.** This service runs in Forge Central,
+  whose CloudWatch logs and AWS metrics (load balancer, ECS, RDS) ship to
+  Grafana; its own metrics don't yet
+  ([FIL-1152](https://linear.app/filecoin-foundation/issue/FIL-1152): Ship
+  metrics from swarf, delegator and piri-signing-service to Grafana). Log a
+  failure an operator would act on at error level, with the context needed to
+  act on it (DIDs and CIDs, never key material), in a stable message an alert
+  can match.
+- **Metrics.** When a feature's health is best read as a number, such as a
+  failure count, a queue depth or the time of the last success, say so in the PR
+  description, naming the metric and
+  [FIL-1152](https://linear.app/filecoin-foundation/issue/FIL-1152), so it can
+  be carried to the issue and instrumented when the service exports metrics.
+- **Alerts and dashboard panels** live in fil-forge/infra-central, under
+  `terraform/envs/grafana/`; its `docs/observability.md` ("Adding an alert or
+  panel") says how to add them. Open a companion PR there, or say in this PR
+  that one is needed.