docs(otel): document the collector in the page the index points at

argus's non-blocking note on #3278: docs/observability.md is what this
repo's reading-paths index names as 'what OTEL options are available',
and it did not mention collector.enable/port/upstreamHeaderName at all.
The nix docstrings covered it, but not where a reader following the
established path would look.

Carries the two things a docstring is a poor home for: that endpoint
keeps meaning 'where telemetry ultimately goes' (the agent-facing value
is derived, so an existing deployment is unaffected), and the
availability trade the collector makes against the direct-export
property this page already promises.

docs/observability.md

@@ -93,6 +93,63 @@ metrics on process exit, so interval tuning is not required for metrics to be
 exported. A lower value gives more frequent intermediate flushes within
 long-running turns — cosmetic, not a correctness knob.
 
+## Running a collector on the host (`otel.collector`)
+
+By default every agent exports **straight to `endpoint`**, which means every
+agent needs `headersCredential` to authenticate — and the harness delivers that
+token into the agent's own `~/.claude/settings.json`, a file the agent can read.
+`0600` protects it from other containers, not from the agent itself.
+
+With `services.hyperhive.otel.collector.enable = true`, a collector runs on the
+host and holds the credential instead. Agents export **unauthenticated** to a
+bridge address only their own containers can reach; the collector adds the
+upstream header and forwards to `endpoint`.
+
+```nix
+services.hyperhive.otel = {
+  enable            = true;
+  endpoint          = "https://collector.example.com/otel";  # still the upstream
+  headersCredential = "/run/secrets/otel-headers";           # now only the host reads it
+  collector.enable  = true;
+};
+```
+
+**`endpoint` keeps meaning "where telemetry ultimately goes."** Turning the
+collector on does not redefine it — the agent-facing value is *derived*
+(`http://<bridgeIp>:<collector.port>`), so an existing deployment's `endpoint`
+keeps working unchanged. The bridge port is contributed to
+`exposeHostPorts` automatically; there is nothing to open by hand.
+
+⚠️ **The trade:** without a collector each harness exports directly, so
+telemetry survives anything host-side being down. A local collector is a new
+dependency in that path. It runs on the same host as the agents, so the window
+is small — but it is not zero.
+
+### `services.hyperhive.otel.collector.enable` — bool, default `false`
+
+Off means *absent*: no unit, no port, and `endpoint` keeps its current meaning
+for every agent. Requires `headersCredential` to be set — a collector with no
+credential is pure indirection, and an assertion says so rather than letting it
+deploy.
+
+### `services.hyperhive.otel.collector.port` — port, default `4318`
+
+The OTLP/HTTP port the collector listens on, bound to the bridge IP only.
+
+### `services.hyperhive.otel.collector.upstreamHeaderName` — string, default `"Authorization"`
+
+Name of the header the collector sends upstream. The **value** comes from the
+credential file at runtime (`EnvironmentFile` → `${env:<name>}`), never from
+nix — so header names are config and header values are secrets, which is the
+only split the collector's static header map can express.
+
+⚠️ **`endpoint` must be valid for `protocol`.** The upstream exporter follows
+`otel.protocol` (`grpc` → the gRPC exporter, otherwise OTLP/HTTP), and the gRPC
+exporter takes an *address*: `https://host/path` is a legal
+`OTEL_EXPORTER_OTLP_ENDPOINT` for HTTP but fails as gRPC with *"missing port in
+address"*. The collector's config is validated at build time, so a mismatch is
+a build error naming the reason rather than telemetry silently going nowhere.
+
 ## Network access
 
 Agent containers can only reach the host on ports 80 and 443 by default. If
@@ -112,6 +169,10 @@ services.hyperhive.otel.endpoint = "http://10.42.0.1:4318";
 The bridge IP is the host's address on the `hvbr0` bridge, typically
 `10.42.0.1`. See `docs/network.md::Reaching host services` for details.
 
+This is the manual form of what `otel.collector.enable` does for you — with the
+collector on, the port is contributed and the endpoint derived, so neither line
+above is needed.
+
 ## Built-in resource labels
 
 Every agent's export includes these resource attributes automatically: