docs: mailctl is retired, the hooks are ours

The user has not run mailctl in months and the coupling this document
described is gone: the live database.hook_dir symlinks post-new,
mailrules.py and qtmaildirconf.py into assets/hooks/ here, where their
three suites also live. All three pass.

The format discipline survives the move and is kept, because what makes
it necessary is two independent readers of one file, not two
repositories: src/tagrules.cpp and assets/hooks/mailrules.py still share
no code and still agree by test. What changes is the procedure around
it, which no longer sends anyone to a sibling checkout, and the round
trip, which is now verified by running the hook rather than by a CLI
that is retired.

Item 166 said the same thing and was filed a day before this was
noticed; its row and its two-repo constraint are corrected with it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HFuRPtzFrSxCQjFk6tq7gD

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