docs: anchor the Version line strip to the generated header

[loonghao]

Summary

--check ignores the Version: line so a new release tag — which changes the
stamped version — does not turn main and every pull request red until someone
regenerates. The previous implementation stripped every line starting with
Version: .

This narrows that to the header only.

Why it matters

Both generated files embed the README and the example scripts. A line inside
that embedded content that happens to begin with Version: is real content,
not the version stamp. Stripping those too meant the comparison could not see
them: a content change touching only such a line would compare equal and pass
the gate silently.

Today each file happens to contain exactly one Version: line, both in the
header, so nothing is currently hidden. The mask would open as soon as embedded
content gained such a line.

The change

The strip now applies only to the header, defined as the lines before the first
## heading, which is where the generator writes its single Version: line.
Everything from the first heading down is compared as-is.

Tests

test/test_generate_llms.py covers both directions, because either alone would
pass under a broken implementation:

• A header-only version change still compares equal (the behaviour we need).
• A Version: line appearing in the body now makes the comparison fail
  (the regression being fixed).
• Only the header line is stripped; body lines are preserved.
• Degenerate documents (no body, empty input) do not raise.

The body case drives is_current against a temporary tree, so it exercises the
path the CLI actually takes rather than only the helper.

Verified against the old implementation: it reports the body change as
not drift, confirming the gap was real and is now closed.

Validation

• python scripts/generate_llms.py --check → up to date, exit 0.
• With an extra release tag (version-only drift) → exit 0, unchanged.
• With a Version: 9.9.9 in body line appended to llms.txt → exit 1.
• ruff check photoshop test scripts and ruff format --check → clean.
• New test file: 6 passed; pytest test_generate_llms.py test_collections.py test_imports.py → 24 passed.

Configuration, generator logic and tests only. No packaging or lockfile
changes, and the generated llms.txt / llms-full.txt are unchanged.