feat(apiproxy): settings/credentials/llm wire domains, frames, and write guard

Eight compiler-locked methods: settings.describe/update/replace serve
redacted layered namespace views (secrets structurally absent from every
layer, write-only in the update direction) and fold seam refusals into
settings-rejected; credentials.describe/set/unset expose value-free views
with credential-rejected on shadowed writes; llm.providers merges the
configurable directory with live routes and llm.models claims the
host-scoped catalog reservation through the buildModelCatalog extraction
session.models now shares. Three HostFrame invalidations bridge the seam
events (host/settings-changed, host/credentials-changed,
host/models-changed), and the connection route generalizes the native-
dialog check into a privileged-method set covering all four writes. The
fixture and both fake clients grow the same face.

packages/host/apiproxy/README.md

@@ -24,6 +24,8 @@ Workspace and Session lists are separate reconnect baselines. `workspace.create`
 
 The `command.*` and `skill.*` domains expose the host command registry and skill catalog to clients. Every method addresses one session's agent by `sessionId` (a served session always has an Agent; `command.*` resumes cold sessions through the same path as `session.*`, while `skill.list` resolves the project root from the session header without touching the Agent registry). `command.execute` runs a slash-command line host-side with pure admission semantics: the response reports whether the line resolved to a handler plus the minted lifecycle `commandId` when it did (correlating the acknowledgment with the flow node), while the outcome rides the durably logged `command/run`/`command/done` lifecycle pair broadcast on the mux stream; the carrier's request signal cancels the running handler. `host/commands-changed` is the catalog invalidation frame: clients refetch `command.list` instead of diffing.
 
+The `settings.*`, `credentials.*`, and `llm.*` domains are the configuration-page wire. `settings.describe` serves every registered namespace with its serialized schemastery schema plus redacted layered values (resolved/`base`/`user` — a field's presence in `user` marks it user-overridden) and the `secrets` slot list; `settings.update`/`settings.replace` write the user layer and answer with the namespace's new redacted view, folding every seam refusal into `settings-rejected`. Secret-role values never ride any response in any layer; a secret crosses the wire in exactly one direction — inside an `update` patch or `credentials.set`. `credentials.describe` returns value-free views (`configured`/`source`/`writable`), and `credentials.set`/`credentials.unset` map a shadowed-reference refusal onto `credential-rejected`. `llm.providers` merges the configurable-provider directory with live routes (dormant entries carry `active: false`; undeclared live routes append with no settings address) and `llm.models` is the session-independent catalog. Three invalidation frames keep every surface converged without polling: `host/settings-changed {ns}` (`settings/updated` passthrough — RPC writes and external `settings.yaml` edits alike), `host/credentials-changed {ref}` (reference names only, never values), and `host/models-changed` (`llm/adapters-updated` passthrough). The browser carrier restricts the four write methods (`settings.update`/`settings.replace`/`credentials.set`/`credentials.unset`) to loopback, same-origin requests — the `host.pickDirectory` privileged set. A composition without a settings or credential provider answers those domains with an actionable `internal` error naming the missing plugin.
+
 ## Carrier layer (`/client` + root)
 
 `AbstractApiClient` holds every protocol invariant — rpcId minting, envelope wrap/unwrap, zod parsing, SSE frame decoding, unary timeout, microtask-batched envelope observation (`subscribeEnvelopes`) — while platform subclasses supply only the `doFetch` transport aspect. `InProcessApiClient` over `toFetchHandler(api)` is the isomorphic point: the full wire serialization/validation path with no network, used by `dsh -p` headless.
@@ -39,6 +41,6 @@ None; this package neither assembles nor sends a provider request.
 ## Known Limitations and Deferred Work
 
 - **`respond` routing is shipped, but pending-interaction state is host-side work** — the wire shape (POST `/api/respond`, `RpcReceipt`) is final; the pending table that makes late/duplicate answers meaningful lives in `src/api-proxy.ts` and is still minimal (questions only, no approvals).
-- **Reserved seams stay out of `RpcMethodMap`** — `session.fork`, `prompt.mode: 'inject'`, `task.list`, `host.listModels`, and a describe `hostInstanceId` are documented reservations; an unknown method fails loud at envelope parse rather than getting a not-implemented code.
+- **Reserved seams stay out of `RpcMethodMap`** — `session.fork`, `prompt.mode: 'inject'`, `task.list`, and a describe `hostInstanceId` are documented reservations (the former `host.listModels` reservation shipped as `llm.models`); an unknown method fails loud at envelope parse rather than getting a not-implemented code.
 - **No protocol version field** — client and host ship together; `host.describe` gains a version negotiation field only when an independently released client exists.
 - **Linux native picker requires desktop tooling** — `host.pickDirectory` reports an actionable error when neither Zenity nor KDialog is installed; it does not fall back to a custom or typed-path browser.