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/26-orchestrating-multiple-agents/lab/agent-prompts/agent-42-count.md

@@ -15,7 +15,7 @@ You are working in this worktree only. Do not touch any other folder.
 
 **Acceptance criteria:**
 
-- `python cli.py count` prints the number of pending tasks and exits 0.
+- `python3 cli.py count` prints the number of pending tasks and exits 0.
 - No other files change. (`README.md`, `CHANGELOG.md`, and `tasks.py` are owned by other agents;
   stay out of them.)
 

modules/26-orchestrating-multiple-agents/lab/agent-prompts/agent-44-clear.md

@@ -17,8 +17,8 @@ You are working in this worktree only. Do not touch any other folder.
 
 **Acceptance criteria:**
 
-- `python cli.py clear` removes all tasks and prints `cleared`.
-- `python cli.py list` afterward shows `(no tasks yet)`.
+- `python3 cli.py clear` removes all tasks and prints `cleared`.
+- `python3 cli.py list` afterward shows `(no tasks yet)`.
 
 When done, commit your work on this branch with a message referencing #44, then push the branch. Stop
 there; the human opens and reviews the PR, and should expect a conflict against `feature/42-count` at