AGENTS.md: add comment deduplication rules

[DAlperin]

Extends the existing "Code comments" section of AGENTS.md with three rules distilled from a comment audit of mz_ore::pool and mz_timely_util::columnar (cleanup in #38434):

• One owning comment per fact. Reasoning lives at the decision point, mechanism docs at the mechanism, config semantics on the public setter or constant; other sites point at the owner or say nothing. Covers the common failure of the same clause appearing near-verbatim in a module doc, a struct doc, and an inline comment, plus its corollaries: don't restate a callee's contract at call sites, and don't duplicate an assert message in a comment.
• Budget doc-comment paragraphs by decisions. No re-arguing a decision documented elsewhere, no enumerating callers or fields, no speculative future-implementation notes, no absence-narration, no ASCII section-divider banners, no unverifiable performance color.
• Test-doc economy. A test whose name and assert messages state the property needs no doc comment; test docs are for non-obvious setup, scenario-encoding fixtures, and multi-phase protocols.

The existing rules in the section (fluff, body narration, chronology, NOTE/TODO) are untouched; the additions only cover what the section didn't already say.

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