docs: drop item 164, the inbox tag on a draft was never there

Re-measured at message level: 0 of 12 drafts carry `inbox`, including
nine written on or before 2026-08-25, when the item was filed. The
thread that produced the original report splits into an arrived message
tagged `inbox` and a draft reply tagged `draft unread`; neither carries
both.

The premise came from `notmuch search --output=tags`, which displays the
union over a thread. The trap has a second half: a thread-level
`notmuch count 'tag:draft and tag:inbox'` also returns 0, because search
terms match per message even in a thread query, so the count and the
displayed tag list disagree and the displayed list is the one that looks
like evidence.

The investigation is kept above the correction rather than deleted: it
cost a week open, two wrong causes and a seven-variant reproducer built
to explain an end state a union produces for free, and it caught a fresh
reader again on 2026-08-27.

The `unread` half of the original observation was real and is item 172,
fixed in edbf393. The reported `draft inbox unread` is fully explained:
`unread` from the missing S flag, `inbox` from the arrived message
sharing the thread.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AtUzfNjMD8fiYfamDd3ywW

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