refactor(client-ui-plugin-config): stage card edits behind an explicit save

Controls committed on blur, which turned leaving a field into a durable,
revision-fenced document write the user could neither preview nor undo, and
silently discarded a draft the field did not accept.

A card's form now owns the staged text every control renders, and Save is the
only point where drafts become writes. Reset stages the composed default the
same way; an invalid draft blocks the save with its reason instead of being
dropped; Discard drops the drafts; a collapsed card marks that it holds some.
The Host stays the only authority on whether a value was accepted, so the save
reads the section back and keeps the drafts of a save that did not land.

packages/client/ui-plugin-config/README.md

@@ -16,7 +16,11 @@ The section declares `settings.plugin.item`, a root list slot. A plugin that shi
 
 ## Writes
 
-Every control writes one field through the client settings scope, which fences each write with the namespace revision it read, so a form that has drifted from the document is refused rather than overwriting a concurrent change. A field's presence in the raw user layer — not its value — is what marks it overridden; a reset clears that field so it re-inherits the composition layer. Secret-role fields never ride a response, so a key control reports only whether one is configured and writes through the credentials domain rather than the settings section.
+A card stages what the user types and writes it only when they save. Each control renders staged text, so what is on screen is exactly what a save would store; **Discard** drops the drafts, and a card holding unsaved edits says so on its header even while collapsed. A reset stages the composed default rather than writing immediately, and a draft the field does not accept blocks the save instead of being dropped.
+
+Saving writes each staged field through the client settings scope, which fences every write with the namespace revision it read, so a form that has drifted from the document is refused rather than overwriting a concurrent change. The Host is the only authority on whether a value was accepted — its validators own the constraints no schema can express — so the card reads the section back afterwards and reports a save that did not land, keeping those drafts for the user to correct.
+
+A field's presence in the raw user layer — not its value — is what marks it overridden; a reset clears that field so it re-inherits the composition layer. Secret-role fields never ride a response, so a key control starts blank, reports only whether one is configured, and writes through the credentials domain rather than the settings section; a blank draft writes nothing and keeps the stored key.
 
 ## Model Experience