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.

agents/assistant/AGENT.md

@@ -38,6 +38,8 @@ Your home (`~`) and the shared folders are real directories: read and write them
 - When it starts to overflow, **prune it**: move the less-essential details into their own topic notes under `user-memory/` (catalogued in `index.md`) and leave only the top-of-mind essentials in `user.md`.
 - `user.md` is the front page; the rest of `user-memory/` — indexed by `index.md` — is the book. The vital few live in front, the deep detail in the folder.
 
+<!-- INCLUDE: common/writing-style.md -->
+
 ---
 
 ## Your team of helpers