Use python3 as the canonical command name course-wide (#104)

Most current systems (default Debian/Ubuntu, recent macOS) install Python
only as `python3`, with no bare `python` on PATH, so learners who copied
`python cli.py ...` into their host shell hit "command not found".

Convert host-shell `python <cmd>` -> `python3 <cmd>` across module/lab
READMEs, lab `.py` docstrings & usage strings, blog posts, lab prompt and
instruction files, the M04 verify.sh message, and the M10/M24 lab patches.
Module 01's convention note (and its blog/02 mirror) is rewritten so
`python3` is canonical and `python` is the documented fallback.

Stop-lines respected: Docker image tags (`python:3.12-slim`), `.venv/.../python`
and `...\.venv\Scripts\python.exe` paths, the M20 `"command": "python"`
teaching example and surrounding venv prose, container-internal invocations
(M16/M18 Dockerfiles, M16 README `docker run` examples), and CI-workflow
`run:` steps fed by `actions/setup-python` / `image: python:3.12` are left
as `python` on purpose.

pip was left out of scope: most occurrences are prose or CI/container-internal,
and `pip3` does not fix the PEP 668 externally-managed-environment refusal that
the course already addresses with venvs. The M01 note is worded to stay
consistent with bare `pip` (use whichever pip pairs with your Python).

Build (tools/build_wiki.py) and tools/check.sh both pass.

Closes #104

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GAEzanEoGJT5o1VizQar47

modules/25-autonomous-agents/lab/agent_runner.py

@@ -14,10 +14,10 @@ not a human watching it type.
 Run it two ways:
 
   1. Simulated (no agent needed, fully deterministic); see the machinery and the gates:
-         python agent_runner.py issue-to-pr issue-delete-command.md --simulate good
-         python agent_runner.py issue-to-pr issue-delete-command.md --simulate bad
-         python agent_runner.py self-heal --simulate bad
-         python agent_runner.py self-heal --simulate stuck
+         python3 agent_runner.py issue-to-pr issue-delete-command.md --simulate good
+         python3 agent_runner.py issue-to-pr issue-delete-command.md --simulate bad
+         python3 agent_runner.py self-heal --simulate bad
+         python3 agent_runner.py self-heal --simulate stuck
 
      Simulation works on a SELF-CONTAINED demo target (agent_demo.py + test_agent_demo.py) so it is
      deterministic and never corrupts your real tasks-app files. The gate it runs (ruff + pytest) is
@@ -26,7 +26,7 @@ Run it two ways:
   2. Real agent: drives your own agentic tool against the actual issue. Point AGENT_CMD at your
      tool's non-interactive / one-shot mode, then drop --simulate:
          export AGENT_CMD='your-agent-cli --print --prompt-file {prompt_file}'
-         python agent_runner.py issue-to-pr issue-delete-command.md
+         python3 agent_runner.py issue-to-pr issue-delete-command.md
 
 Language: Python 3.10+. Standard library only.
 """