feat(fleet): enforce generated environment boundary (#772)

packages/mosaic/framework/fleet/README.md

@@ -9,7 +9,8 @@ package, normally at:
 ```
 
 The default tmux socket is `mosaic-fleet` so fleet commands do not touch the
-default tmux server.
+default tmux server. The roster is the desired-state authority; generated environment files are
+rebuildable projections, never a second source of configuration.
 
 ## Examples
 
@@ -31,6 +32,17 @@ The installed `tools/fleet/print-interaction-effective-policy.sh` prints only
 the resolved name, runtime, model, reasoning, and tool policy. It never reads
 or prints credential variables.
 
+## Generated agent environment boundary
+
+`mosaic fleet install` writes a private deterministic projection at
+`~/.config/mosaic/fleet/agents/<agent>.env.generated`. It may relocate only approved local machine
+data to `<agent>.env.local`; generated keys, arbitrary commands, secret-like keys, duplicate keys,
+unknown keys, and unsafe permissions fail before a tmux session is created. Legacy `.env` input is
+regenerated, relocated, or quarantined and is not a launch authority.
+
+See [`docs/fleet/reference/generated-env-boundary.md`](../../../../docs/fleet/reference/generated-env-boundary.md)
+for allowed local keys and the USC downstream interface evidence.
+
 Initialize a roster:
 
 ```bash