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.

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.

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.

Add a host prerequisite section to both bootstrap guide copies.
Include python-is-python3 in the Debian/Ubuntu package list with an issue comment.
Note the non-Debian equivalent as a /usr/bin/python -> python3 symlink.
Validate markdown/diff, run shell syntax checks where applicable, run required review, commit, queue guard, and push.

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.