feat: adapt to DSH 0.1.7-alpha.1 and yield the read-only previews

DSH 0.1.7-alpha.1 is 1299 commits past 0.1.6-alpha.2 and rewrites four
contracts this plugin was built on. The peer floor moves to
^0.1.7-alpha.1 (semver's prerelease rule means ^0.1.6-alpha.2 can never
match a 0.1.7 prerelease); 0.1.6-alpha.2 and earlier stay on v0.19.1.
Released as 0.21.0-alpha.1 (dist-tag alpha).

Hard breaks:
- dsh-settings was rewritten: register(ns, schema) is gone, a form is
  addressed by the plugin row's Loader entry id and read from the
  module's exported Config. The preferences are merged into Config and
  marked volatile — which requires DSH's own schemastery, because only
  that build wraps a volatile field in the cosmokit reference the
  loader's volatile commit walks. With the public package a settings
  write reports success and revision 1 while the effective value never
  changes.
- ui-primitives renamed its whole icon family (IconXxx14/16 ->
  IconXxxRegular/Medium): 26 imported symbols, 17 files. The failing
  mode is render-time, not load-time. Five call sites that relied on the
  old 14px default now pass size={14} explicitly.
- Session format v4 rejects source.kind === 'plugin'; the side-chat
  boundary injection now writes plugin:dsh-better-sidebar, the same kind
  DSH's own v3->v4 migration produces, so historical threads survive.
  Tool results moved to role 'tool' with top-level content/isError; all
  four parsers accept both shapes.
- The client session snapshot dropped subagentsByParent/jobsBySession
  and the on-demand subagent-catalog observation API. The Tasks page
  reads projectionsBySession and a new jobs.list route.

Yields and convergence:
- The read-only previews go to DSH's documentpreview (Office->PDF, a
  spreadsheet worker, zoom, per-directory refresh); editor.canOpen
  refuses those 26 extensions. Markdown, HTML (with its sandbox
  switches) and the editable code buffer stay.
- The host browser kind is desktop-profile-only now, so the web e2e
  lanes expect no browser entry and assert its absence.
- Link takeover claims only urlTarget links and passes the rest to the
  host, fixing a regression from the 0.1.6 round: the fallback pointed
  at a 'browser' type the plugin no longer registers, so http links in
  its own markdown did nothing.

Also fixed, found along the way:
- The file tree refreshes live (new directory watch over the plugin's
  own socket), since the plugin owns the `files` kind.
- ctx.sessions.list has no `current` field at either tag; the three
  readers were dead. They now use ctx.sidebarRight.mounted.
- package.json.files was missing lib/client-locale.js.
- Two genuinely dead call sites: sessions.openSubagent/open/
  refreshSubagents do not exist on the current baseline, so the
  topology click, the jump-back and the retry were all no-ops.
- The test harness window shim lacked addEventListener, which the 0.1.7
  primitives call at module-evaluation time; and the e2e token exchange
  needs Playwright's request stack, not Node's fetch.

src/prefs-shared.ts

@@ -110,30 +110,6 @@ export interface SidebarPrefs {
    * restore for the current file.
    */
   htmlViewerDefaultUnsafe: boolean
-  /**
-   * MASTER switch: whether clicking an EXTERNAL link in the GUI (chat
-   * messages, tool rows, prose mentions) is taken over into the sidebar at
-   * all. On by default; the per-protocol granularity lives in
-   * `browserInterceptHttp` / `browserInterceptHttps` (the protocol flag
-   * must also be on). Ctrl/Cmd+click always bypasses the takeover. Kept as the master so old
-   * documents keep their meaning with no migration (an explicit `false`
-   * stays "never take over").
-   */
-  browserInterceptLinks: boolean
-  /**
-   * Whether clicking an http EXTERNAL link in the GUI opens DSH's own
-   * sidebar browser (or a plugin tab that declares `urlTarget`) instead of a
-   * new browser tab. On by default; gated on the `browserInterceptLinks`
-   * master.
-   */
-  browserInterceptHttp: boolean
-  /**
-   * Whether clicking an https EXTERNAL link in the GUI opens the sidebar
-   * instead of a new browser tab. OFF by default — most https sites (e.g.
-   * GitHub) refuse iframe embedding, so the system browser is the smoother
-   * default; gated on the `browserInterceptLinks` master.
-   */
-  browserInterceptHttps: boolean
   /**
    * Per-tab enable switches, keyed by tab descriptor id (`'explorer'`,
    * `'my-plugin:db'`). An ABSENT key means enabled — only an explicit
@@ -185,9 +161,6 @@ export const SIDEBAR_PREFS_DEFAULTS: SidebarPrefs = {
   titleBarStripPx: TITLE_BAR_STRIP_DEFAULT,
   htmlViewerNoSandbox: false,
   htmlViewerDefaultUnsafe: false,
-  browserInterceptLinks: true,
-  browserInterceptHttp: true,
-  browserInterceptHttps: false,
   tabsEnabled: {},
   viewersEnabled: {},
   pluginSettings: {},