refactor(client): command decorations replace the hostBacked contribution mode

A popup on a host command is not a second command — it is what that
command's BARE invocation does on this client. CommandContribution loses
hostBacked (contributions are pure client commands again; a host-name
collision fails loud, unchanged for /model), and the contract gains
CommandDecoration + command.decorate(): key = the HOST command name, no
catalog row, no claim participation. Dispatch consults decorations only on
the bare paths (menu pick / bare enter) after the host row resolves; space
and argued enter never see them — the two edges hostBacked had to guard
explicitly hold by construction in the decoration model. A decorated name
with no host row in the session's directory never fires (a decoration
cannot manufacture a command).

ui-permission switches register→decorate with zero behavior change
(options still read the permissions projection; a pick still submits
'/permission <preset>'). Specs rewrite to the decoration semantics: no
catalog row, bare-enter popup vs argued-enter host claim, space host
claim, no-host-row miss, unavailable fall-through, duplicate fail-loud.

packages/client/ui-command/README.md

@@ -4,7 +4,7 @@ English | [中文](README.zh.md)
 
 Client command surface (`ctx.command`): the session-keyed command-directory cache, the `/` command source with matchSpace/matchEnter adjudication hooks, three-kind dispatch (execute / popupSelect / leadingInput), and the popupSelect registration face for business packages. Contract: the [web command surfaces Agent Note](../../../.agents/notes/implemented/architecture/2026-07-25-web-command-surfaces-and-assembly.zh.md).
 
-`src/client/contract.ts` is the frozen business face: `CommandServiceContract.register(name, spec)` is everything a business package consumes; `CommandUiSpec{options, onSelect}` keeps popup data self-served — the shell component is this package's and business never sees it. Command kinds derive per dispatch, never per registration: a host descriptor with `input` is leadingInput, a registered `CommandUiSpec` is popupSelect, everything else is execute.
+`src/client/contract.ts` is the frozen business face: `CommandServiceContract.register(name, spec)` and `decorate(name, spec)` are everything a business package consumes; `CommandUiSpec{options, onSelect}` keeps popup data self-served — the shell component is this package's and business never sees it. A contribution is a client-owned command (a host-name collision fails loud); a decoration hangs a bare-invocation popup on an EXISTING host command — the host keeps its catalog row, argument claim (space / argued enter), and lifecycle logging, and a decorated name with no host row in the session's directory simply never fires. Command kinds derive per dispatch, never per registration: a host descriptor with `input` is leadingInput, a registered `CommandUiSpec` is popupSelect, everything else is execute.
 
 `CommandDirectory` (`src/client/directory.ts`) is the one wire-derived cache, keyed by session: every session is agent-backed, so `command.list({sessionId})` is the only address shape and the source's scope-birth `warm` hook prewarms the session's entry. Entries are soft-invalidated by the `commands/changed` typed event (old snapshot serves while the repull flies), hard-invalidated by `connection/reset`, epoch-guarded so a superseded pull can never overwrite a newer one. `matchSpace` answers synchronously from this cache only; `matchEnter` strong-waits it on the SubmitAttempt signal and rejects on warmup failure — a `/` line is never silently downgraded to a plain prompt.