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.

.agents/notes/implemented/feature/2026-08-10-web-plugin-configuration.md

@@ -26,6 +26,8 @@ Three host-plane plugins register their own settings namespace, and one browser-
 
 **The section knows no namespace.** `dsh-client-ui-plugin-config` declares a `settings.plugin.item` slot and renders the cards registered into it, so a plugin that ships a browser half owns its card and its controls. Each card binds its namespace through the client settings scope, which gained the two things a form needs: the raw `user` layer, whose key PRESENCE is what marks a field overridden, and `unset`, which clears one field back to the composition layer. A card renders nothing while its namespace is unavailable, so a deployment that does not compose the owning plugin shows no trace of it.
 
+**A card stages its edits and writes them on save.** Controls hold no draft of their own: the card's form owns the staged text, every control renders it, and only **Save** turns it into document mutations. A settings write is durable and revision-fenced, so a control that committed as it settled spent a revision on a value the user had not decided to store and could not preview; the reset stages the composed default the same way. Because the Host's validators own the constraints no schema can express, the form reads the section back after writing and reports a save that did not land instead of predicting the outcome, keeping those drafts for the user to correct. The credential control is staged with the rest even though it writes through the credentials domain, so one save covers everything the card shows.
+
 ## Alternatives considered
 
 - **A registration-time exposure declaration replacing the allowlist.** The honest shape — the namespace's owner declares its own exposure, and a plugin distributed outside this repository can surface its configuration without a change in `packages/host/apiproxy`. Deferred because it changes the seam contract, every existing registration site, and the anti-enumeration semantics at once, and because a plugin exposing an arbitrary schema needs a fail-closed redaction path first: a secret reachable only through a union or transform is currently returned verbatim.
@@ -33,6 +35,8 @@ Three host-plane plugins register their own settings namespace, and one browser-
 - **Editing preset-mounted plugins from this page.** Out of scope, and not merely unbuilt: a preset's rows carry their configuration inline in `agent.cordis.yml` and cannot register a settings namespace at all, because a second session mounting the same preset would fail on a duplicate registration. A user layer shared across presets would also overwrite the fields a preset uses to define its agent's identity — its persona text, its delegation wiring — which are per-preset by design.
 - **One namespace per executor package instead of the capability-named `bash`.** Declined because the composed executor differs by platform while the settings document does not: a user who set a timeout on macOS would silently lose it on Windows.
 - **Writing the search key into the settings section.** Declined because the literal would then have to ride a `describe` response to be rendered. The card reports only whether a key is configured and writes through the credentials domain, addressed by the reference the section names.
+- **Committing each control as it settles, with no save.** Built first, and replaced: blur is not a decision. It spent a namespace revision per control, gave the user nothing to preview or undo before the write, and left an invalid draft silently discarded — a value the Host's validator refuses simply snapped back with no reason given. One save per card makes the write a gesture the user performs.
+- **Validating the fields in the browser to keep the save honest.** Declined: the constraints live in the owning plugin's section validator, and restating them here would make two homes for one rule that could disagree per release. The card checks only what its own control can decide — that a numeric draft is a number — and lets the Host answer for the rest, which is why the save reads the section back.
 
 ## Consequences