fix(host,client): gate the picker affordance on the advertised kind; reject non-absolute browse paths

ds-review-bot round 2. The workspace UI never consulted the advertised
directoryPicker kind: under a browse (or merge-added) backend it still
rendered 'Open local folder…' and called pickDirectory(), which the host
answers with directory-picker-unavailable. The create flow now reads
directoryPickerKind() per menu open and renders the dialog affordance only
under 'dialog' — browse (until its in-app browser UI lands) and unknown
kinds hide the entry, realizing the seam's documented default; a keyless
workspace-flow snapshot pins the hidden entry over the browse fixture.

The browse backend also resolved wire paths, silently rebasing '' or
relative parents under the host process cwd; both primitives now reject
non-absolute explicit paths with their business codes, and the seam JSDoc
carries the contract.

packages/host/directory-picker/src/index.ts

@@ -60,7 +60,8 @@ export interface DirectoryPickerBrowseCapability {
    * List one directory level.
    * @param path - absolute directory to list; absent lists the home directory.
    * @returns the level's listing with ancestry.
-   * @throws {DirectoryPickerError} `directory-unreadable` when the target cannot be listed.
+   * @throws {DirectoryPickerError} `directory-unreadable` when the target is not absolute
+   * (a wire value must never rebase under the host cwd) or cannot be listed.
    */
   list(path?: string): Promise<DirectoryListing>
   /**
@@ -68,7 +69,8 @@ export interface DirectoryPickerBrowseCapability {
    * @param path - absolute existing parent directory.
    * @param name - single non-blank path segment (no separators, not `.`/`..`).
    * @returns the created directory's absolute path.
-   * @throws {DirectoryPickerError} `directory-exists` for an existing child, `directory-create-failed` otherwise.
+   * @throws {DirectoryPickerError} `directory-exists` for an existing child,
+   * `directory-create-failed` for a non-absolute parent or any other failure.
    */
   createDirectory(path: string, name: string): Promise<string>
 }