Chores and bugfixes: domain references, CI trigger, docs nits

[Xarlos89]

A batch of small fixes that came out of looking over the project at main. Nothing here really changes site content in any meaningful way; it's mostly things that were wrong, stale or slightly broken.

Fixes

• Wrong domain. docs/CNAME, the README title and the User-Agent in
  scripts/refresh_member_count.py all said practical-python.org, which doesn't exist. The site
  lives at practicalpython.org.
• CI now runs on every pull request. It was limited to PRs targeting main, so PRs into feature
  branches (like the tutorial chapters) were merging with no checks at all.
• Broken tabs on the Channels page. The "It's a long list..." tip wasn't indented, which split
  the bot commands section into two separate one-tab groups. It now sits inside the "Slash commands" tab and the two tabs render together.
• Makefile .PHONY. The targets were comma-separated, so Make was treating run, and strict, as the phony names. Commas removed.

Docs cleanup

• README layout tree now includes static/, python-tutorial/ and code-adventure/.
• README no longer claims a requirements.txt is generated becasue there isn't one in the repo.
• CONTRIBUTING lists member_count alongside the other variables.
• Removed the stale logo TODO from zensical.toml.
• Spelling: A few small, random spelling mistakes.

Testing

uv run zensical build --strict passes. No pages added, removed or renamed.