feat(agents): learn and reuse the user's writing style

The chat agents now treat a stated preference about how something should be
written — or a correction to a draft they produced — as durable, and record it
in a `## Writing style` section of `user-memory/user.md`: preferred wording,
how emails open and close, what changes between the formal and the informal
register, and contacts written to differently from everyone else.

The section is capped at 10 lines, because it shares `user.md`'s own 40-line
budget and per-recipient detail is the half that grows without bound; past
that, the whole section moves into its own note and leaves a pointer behind.
A rule that turns out to be wrong is corrected in place rather than joined by
a second bullet contradicting it.

Shipped as the shared fragment `common/writing-style.md`, included by the
three `type: chat` agents. Only `assistant` injects `user.md` automatically,
so the fragment closes by telling the other two to read it before drafting.

CHANGELOG.md

@@ -19,6 +19,9 @@ release PR may merge — and a section is closed at the commit that bumps it.
 - The agent is told what its sandbox can actually run, from a probe of its own container.
 - Event triage can be tuned per person: a check interval that overrides the instance one,
   and notification preferences read from `user-memory/notifications.md`.
+- The assistant now remembers how you like emails and documents written — preferred
+  wording, openings, sign-offs, formal vs. informal, per-recipient exceptions — as a short
+  section of your private `user.md`, and applies it to later drafts.
 - File viewer: syntax highlighting for code files and for code blocks in the chat, a
   hover copy button on those blocks, and history browsing for a file under git.
 - Project explorer: download a folder as a streaming ZIP.