docs(llms.txt): update dirhtml-skip rationale after .md pivot

The previous comment on `_write_llms_txt` (and on the LLM-Facing Files
section in CLAUDE.md) justified the `dirhtml` skip by claiming the
template's `.html` URLs wouldn't resolve under `dirhtml` output. That
became stale after [#1874@5fe7f9ad](https://github.com/vyos/vyos-documentation/pull/1874/commits/5fe7f9ad) (and parity commits on
[#1876](https://github.com/vyos/vyos-documentation/pull/1876)/[#1903](https://github.com/vyos/vyos-documentation/pull/1903)) pivoted the curated template to `.md` URLs —
`_copy_md_sources` puts `.md` files at their source-relative paths
regardless of builder, so the rendered llms.txt would actually resolve
fine under `dirhtml` now.

The real reason we skip `dirhtml` is simpler: production publishes
only via the `html`/`readthedocs` builders, and we don't render
llms.txt for builds we don't ship. Local `make dirhtml` is a developer
convenience, not a publish target.

Reword the conf.py comment and the CLAUDE.md section to say that
plainly. No code-behavior change.

Addresses Copilot review feedback on PR #1874 (3 threads on docs/conf.py,
docs/_templates/llms.txt.j2, and CLAUDE.md flagging the inconsistency).

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