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.ts

@@ -0,0 +1,44 @@
+/**
+ * credentials domain contract: the web face of the credential-reference seam
+ * (`ctx.credentials`). Reads are structurally value-free — a credential view
+ * carries configured/source/writable and has no slot for the value — and the
+ * value crosses the wire in exactly one direction, inside `credentials.set`.
+ * There is no enumeration method by design: clients learn which references
+ * exist from settings schemas and values (`apiKeyEnv` fields).
+ */
+
+import type { RpcRequest, RpcResponse } from './rpc.ts'
+
+/** Wire view of one credential reference's state. */
+export interface CredentialView {
+  /** Whether any layer currently supplies a non-empty value. */
+  configured: boolean
+  /** Winning layer when configured (`env`, `file`, …); provider vocabulary. */
+  source?: string
+  /** Whether `credentials.set`/`credentials.unset` can affect this reference. */
+  writable: boolean
+}
+
+/** Credentials-domain unary methods (the map keys credentials.* of RpcMethodMap). */
+export interface CredentialsApi {
+  /**
+   * Describe the named references (batch): configured state, winning source,
+   * and writability — never values. An invalid reference name is a
+   * `bad-request`; an unknown-but-valid one describes as unconfigured.
+   */
+  describe(request: RpcRequest<{ refs: string[] }>): Promise<RpcResponse<{ credentials: Record<string, CredentialView> }>>
+
+  /**
+   * Store one credential value in the writable layer. Rejected with
+   * `credential-rejected` while a read-only layer (the live environment)
+   * shadows the reference — the write would otherwise appear to succeed while
+   * resolution keeps returning the shadowing value.
+   */
+  set(request: RpcRequest<{ ref: string; value: string }>): Promise<RpcResponse<{}>>
+
+  /**
+   * Remove one credential from the writable layer; same shadowing rejection
+   * as `set`. Unsetting an absent reference succeeds (idempotent).
+   */
+  unset(request: RpcRequest<{ ref: string }>): Promise<RpcResponse<{}>>
+}