core: make the wallet an API, not a user-facing feature

Names commands will use the wallet; users do not. The commands move to
/_wallet, and the help section, its topic and the changelog entry are
removed. The tests drive the API end to end.

Comments cut to what the code does not say, dead exports removed.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Rvc3HbiWBTqbAvRT45G5oX

src/Simplex/Chat/Store/Postgres/Migrations/M20260908_wallet_seeds.hs

@@ -12,9 +12,9 @@ m20260908_wallet_seeds =
 CREATE TABLE wallet_seeds (
   wallet_seed_id BIGINT GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
   seed BYTEA NOT NULL,
-  -- Known issue: see the SQLite migration.
+  -- see the SQLite migration
   next_account_index BIGINT NOT NULL DEFAULT 0,
-  -- one key per device for now; drop when several are supported
+  -- one key per device for now
   single_seed SMALLINT NOT NULL DEFAULT 1 UNIQUE
 );