docs: add qtmaildir v1 implementation plan

Fourteen TDD tasks building from pure-logic units outward: keymap, config,
MIME parsing, and the request interceptor are fully tested first, then the
notmuch layer, then the UI that wires them together.

Seven test binaries, up from the three the spec named; config, htmlbuilder,
thread model, and sync each earned one. NotmuchWorker stays untested by
design, with Task 13 as the compensating manual checklist against a real
database.

Two narrowings from the spec are recorded rather than silently dropped:
thread rendering shows the newest message instead of a flat list of all
messages, and tagging applies to the loaded thread rather than every
selected one.

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

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