docs(bootstrap): add python-is-python3 host prereq (#561)

guides/BOOTSTRAP.md

@@ -15,6 +15,22 @@ This guide covers how to bootstrap a project so AI agents (Claude, Codex, etc.)
 7. Branching/merging is consistent: `branch -> main` via PR with squash-only merges
 8. Steered-autonomy execution is enabled so agents can run end-to-end with escalation-only human intervention
 
+## Agent Host Prerequisites
+
+Agent hosts must provide the Python runtime shape that runtime agents and
+Mosaic automation assume is present.
+
+For Debian/Ubuntu hosts:
+
+```bash
+sudo apt-get update
+# #561: bare python invocations from agents must resolve.
+sudo apt-get install -y python3 python-is-python3
+```
+
+For non-Debian hosts, install the equivalent Python 3 runtime and ensure
+`/usr/bin/python` resolves to `python3` (for example, via a managed symlink).
+
 ## Quick Start
 
 ```bash