fix: add numpy install sections to L1–L4 answers, close text-notebooks TODO, fix AGENTS.md

[mmmarinho]

Summary

This PR was created by an AI agent (OpenHands) on behalf of the user.

1. Missing "Package installation" section in L1–L4 exercise answers

The L1, L2, L3 and L4 exercise answers were missing the Package installation section
that every other lesson has (see e.g. L5 exercise answers and the L2–L5 tutorials).
Each file now contains:

# Package installation

````{code-cell}
%%capture
%pip install numpy

`numpy` is installed (rather than `numpy matplotlib`) because L1–L4 do not use
matplotlib — matching the L2/L3/L4 tutorials, which also install only `numpy`.

### 2. TODO.md — text-based notebooks achieved

The "Text-based notebooks" section of `TODO.md` is now marked as **done — achieved in
August 2026**, with a pointer to PR #9 (the migration from classic `.ipynb` notebooks
to MyST text notebooks under `basic_lessons/`). The two completed bullets are crossed
out with ~~strikethrough~~ and kept for the record.

### 3. AGENTS.md — small accuracy fix

The "Files excluded from version control" section claimed `venv/` and `_build/` are
excluded from git, but no root `.gitignore` exists (and no one has ever been tracked).
The section now only lists `basic_lessons/*.ipynb` (which is actually ignored via
`basic_lessons/.gitignore`) and notes the absence of a root `.gitignore`.

The rest of AGENTS.md was checked against the current repository state (post PR #9):
structure, build commands, workflow, header convention and lesson-adding steps are all
still accurate.

## Verification

- All edited `.md` files round-trip through `jupytext --from md:myst --to notebook`
  without errors.
- The added section matches the existing `# Package installation` pattern used in the
  tutorial lessons exactly (heading level, blank lines, `%%capture`).

[mmmarinho]

👍