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

docs/scratchpads/561-python-is-python3.md

@@ -0,0 +1,33 @@
+# Issue #561 — Bare python on agent hosts
+
+## Objective
+
+Make the durable bootstrap/provisioning guidance ensure agent hosts provide a bare `python` command that resolves to Python 3.
+
+## Scope
+
+- Add Debian/Ubuntu `python-is-python3` to agent-host prerequisites in bootstrap docs.
+- Check for actual OS package provisioning scripts and update only if an existing agent-host package install path exists.
+- Do not touch live host state.
+- Do not update `docs/TASKS.md`; repo guidance says workers read it but never modify it.
+
+## Recon
+
+- Issue #561 confirms repeated `python: command not found` failures from fleet agents that emit `python foo.py`.
+- `guides/BOOTSTRAP.md` and `packages/mosaic/framework/guides/BOOTSTRAP.md` are the source and packaged framework copies of the bootstrap guide.
+- Targeted repo sweep found no agent-host Debian package provisioning script. Existing `apt-get install` hits are CI/test helper paths or unrelated deployment docs.
+
+## Plan
+
+1. Add a host prerequisite section to both bootstrap guide copies.
+2. Include `python-is-python3` in the Debian/Ubuntu package list with an issue comment.
+3. Note the non-Debian equivalent as a `/usr/bin/python -> python3` symlink.
+4. Validate markdown/diff, run shell syntax checks where applicable, run required review, commit, queue guard, and push.
+
+## Validation Log
+
+- `rg` recon: no existing agent-host Debian package provisioning script; only CI/test helper `apt-get install` paths and unrelated deployment docs.
+- `git diff --check`: passed.
+- `bash -n packages/mosaic/framework/install.sh tools/install.sh packages/mosaic/framework/tools/bootstrap/init-project.sh packages/mosaic/framework/tools/_scripts/mosaic-bootstrap-repo`: passed. No touched shell scripts.
+- `~/.config/mosaic/tools/codex/codex-code-review.sh --uncommitted`: approved, 0 findings.
+- `pnpm format:check`: initially blocked because `node_modules` was absent and `prettier` was unavailable; `pnpm install --frozen-lockfile` initially hit an invalid `/root` pnpm store path. Reran install with `--store-dir /home/hermes/agent-work/.pnpm-store`, then `pnpm format:check` passed.

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

packages/mosaic/framework/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