ci(doc-linter): delete lint_AS placeholder

`lint_AS()` and its `NUMBER` regex were a placeholder for a
future AS-number documentation-range check (RFC 5398).
`lint_AS()` was never called from anywhere — it'd merely
`pass` on `re.search` hit. Pure dead code that made it look
like AS-number linting existed when it didn't.

Delete:
- the `NUMBER` regex constant
- the `lint_AS()` function

If/when AS-number linting is actually desired, implement it
properly: hook into the lint loop in `handle_file_action()`,
return the standard `(message, line, severity)` tuple on
violations, and define the allowed AS ranges from
`/^AGENTS.md/` (currently 64496–64511 and 65536–65551).

Tracked as item 8 of the rolling-side cleanup backlog from
PR #2014 / #2019 / #2020 reviews.

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(cherry picked from commit 61ecb4e103c6a6225b3d71586f7f65e41c3e3510)

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