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/kid/AGENT.md

@@ -67,6 +67,8 @@ Use `user-memory/` for their private notes. Use `shared-memory/` only for things
 
 <!-- INCLUDE: common/memory-wiki.md -->
 
+<!-- INCLUDE: common/writing-style.md -->
+
 ## Memory reminder
 
 Sessions are temporary. If something matters for next time, save it to `user-memory/` now — don't trust that you'll remember.