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/src/api/credentials.schema.ts

@@ -0,0 +1,48 @@
+/**
+ * credentials domain zod schemas (names derived from map keys:
+ * credentialsDescribeRequestSchema / credentialsDescribeValueSchema / …).
+ * The reference-name pattern mirrors the seam's `credentialRef` guard so an
+ * invalid name fails as `bad-request` before reaching the service.
+ */
+
+import { z } from 'zod'
+import type { RequestPayload, ResponseValue } from './rpc-map.ts'
+import type { Wire } from './rpc.schema.ts'
+import type { CredentialView } from './credentials.ts'
+
+/** POSIX-portable environment-variable name (the seam's `credentialRef` pattern). */
+export const credentialRefNameSchema = z.string().regex(/^[A-Za-z_][A-Za-z0-9_]*$/)
+
+/** CredentialView entry of credentials.describe. */
+export const credentialViewSchema = z.object({
+  configured: z.boolean(),
+  source: z.string().optional(),
+  writable: z.boolean(),
+}) satisfies z.ZodType<Wire<CredentialView>>
+
+/** credentials.describe request payload. */
+export const credentialsDescribeRequestSchema = z.object({
+  refs: z.array(credentialRefNameSchema).max(64),
+}) satisfies z.ZodType<Wire<RequestPayload<'credentials.describe'>>>
+
+/** credentials.describe response value. */
+export const credentialsDescribeValueSchema = z.object({
+  credentials: z.record(z.string(), credentialViewSchema),
+}) satisfies z.ZodType<Wire<ResponseValue<'credentials.describe'>>>
+
+/** credentials.set request payload: the one direction a value crosses this wire. */
+export const credentialsSetRequestSchema = z.object({
+  ref: credentialRefNameSchema,
+  value: z.string().min(1),
+}) satisfies z.ZodType<Wire<RequestPayload<'credentials.set'>>>
+
+/** credentials.set response value. */
+export const credentialsSetValueSchema = z.object({}) satisfies z.ZodType<Wire<ResponseValue<'credentials.set'>>>
+
+/** credentials.unset request payload. */
+export const credentialsUnsetRequestSchema = z.object({
+  ref: credentialRefNameSchema,
+}) satisfies z.ZodType<Wire<RequestPayload<'credentials.unset'>>>
+
+/** credentials.unset response value. */
+export const credentialsUnsetValueSchema = z.object({}) satisfies z.ZodType<Wire<ResponseValue<'credentials.unset'>>>