docs(plans): implementation plan for the shared tagging rules

Fifteen tasks across two repositories, TDD throughout. Tasks 1-9 build
the format, the post-new hook and mailctl's read-only rules command;
10-13 add qtmaildir's reader and dialog; 14 migrates the real rules and
15 closes the backlog item.

One deviation from the spec, recorded at the top of the plan: the spec
called for a countRules worker slot, but requestCounts already exists
and counts threads. A tagging rule tags messages, so the plan adds a
general requestMessageCounts instead of a rule-specific slot.

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