docs: authenticated telemetry ingest, and where the hive's secret comes from

Three readers, three different questions:

`observability.md` gets the operator's version — what to set (nothing, on an
all-local swarm), what to set elsewhere, what a misconfiguration looks like
(401s and a missing hive), and what turning it off actually costs.

`swarm/secrets.md` extends the existing client-secret row rather than adding
one, because this is a new *reader* of a secret already described. It notes the
one thing that makes this reader unusual: a host unit reads the file in place,
so unlike every other consumer there is no delivery step to get wrong.

`setup.md` gets one bullet under security notes, since a first-run hive gets
the property without doing anything and should know it has it.

docs/setup.md

@@ -156,6 +156,11 @@ See [`tools/hivectl.md`](tools/hivectl.md) for every `hivectl` verb.
 - All config changes (forge PRs on `agent-configs/<name>`) go through
   operator approval — agents can't unilaterally rebuild containers, by design.
   See [`boundary.md`](boundary.md) and [`security.md`](security.md).
+- **Telemetry ingest is authenticated per hive**, and the `hive` label comes
+  from which hive authenticated rather than from the payload — so no hive can
+  report metrics as another. A first-run all-local hive gets this with nothing
+  to configure; joining a swarm you don't host needs one secret copied across.
+  See [`observability.md`](observability.md#authenticated-ingest).
 
 Once the hive is running, ruth records anything it needs to remember
 across restarts in `/agents/ruth/state/notes.md`.