Extract reverse-DNS map guidance to a nested AGENTS.md

Move the "Maintaining the reverse DNS maps" section (two-thirds of the
root AGENTS.md) to parsedmarc/resources/maps/AGENTS.md, next to the
files and tooling it governs. Agents that follow the AGENTS.md spec
read the nearest file in the directory tree; a one-line CLAUDE.md
import alongside it covers Claude Code, which loads subdirectory
context lazily when files under maps/ are first read. A pointer stub
remains in the root file for agents that only read the repo root.

The moved content is verbatim except for one cross-reference that now
points at the root AGENTS.md instead of "earlier in this file".

Exclude AGENTS.md/CLAUDE.md from hatch build targets so the new files
don't ship in the wheel, matching the existing README.md exclusion
(verified with a scratch wheel build).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

pyproject.toml

@@ -115,6 +115,8 @@ exclude = [
     "base_reverse_dns.csv",
     "unknown_base_reverse_dns.csv",
     "README.md",
+    "AGENTS.md",
+    "CLAUDE.md",
     "*.bak",
     # Maintenance tooling: any Python file under parsedmarc/resources/maps/
     # whose name doesn't start with `_` (i.e. everything except __init__.py,