docs: correct the lab-notes section — no auto-writer

[adilfaisal01]

The Lab Notebooks section claimed a CI bot (.github/workflows/update-labnotes.yml) appends commit logs to the day's note on push to main. That is not true and has not been true for a while:

• The workflow has failed on every run since Sep 2 — GH006: Protected branch update failed ... Changes must be made through a pull request. Branch protection blocks a direct bot push to main. It is not a required check, so nothing surfaced the failures.
• The actual writer was a local .git/hooks/pre-commit that appended timestamped headers and force-staged the note. Local hooks live in .git/, so they are untracked and invisible to review and CI. That hook has now been removed.

Both had the same two defects: the filename used the local date while the header used UTC (evening commits wrote a next-day header into that day's file), and the commit range always collapsed to empty, leaving hollow headers (57% of all headers had nothing under them).

So the single source of truth is now the agent writing the summary at the end of the task. Nothing appends automatically.