docs(orchestrator): tighten auth-provisioning wording to concrete services

Address review on #482: drop the generic "credential boundary" framing in the
PRD and docstrings and talk about the specific services — the orchestrator holds
the control-plane key; the host controller (#468) gets a separate key the
orchestrator never holds, so the orchestrator can't mint the credentials it uses
to talk to the host controller that owns its lifecycle. Lead with that concrete
win. No code behaviour change.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

bot_bottle/paths.py

@@ -101,14 +101,12 @@ def host_signing_key(filename: str) -> str:
     """A per-host signing key at `<root>/<filename>`, minted (256-bit, url-safe)
     and persisted 0600 on first use, then reused.
 
-    The generic form of `host_orchestrator_token()`: a *trust domain*
-    (`trust_domain.py`) names its own key file so each credential boundary gets a
-    distinct host-canonical key — the orchestrator control plane names one file,
-    a separate boundary (e.g. a host controller) names another, and neither can
-    read the other's key (issues #476/#468). It is a *host* artifact: the file
-    lives under the root the agent never mounts, and its value is injected only
-    into the trusted control-plane process, so reading it here is safe on the
-    host launch path but the value never reaches a bottle."""
+    The generic form of `host_orchestrator_token()`: each service names its own
+    key file (`trust_domain.py`), so the orchestrator and a separate service like
+    the host controller (#468) get distinct keys neither can read. It is a *host*
+    artifact — the file lives under the root the agent never mounts, and its value
+    is injected only into the trusted control-plane process — so reading it here
+    is safe on the launch path but the value never reaches a bottle."""
     path = bot_bottle_root() / filename
     try:
         existing = path.read_text().strip()