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/client/service.ts

@@ -208,16 +208,15 @@ export interface TabDescriptor {
   createTab?: (state: SidebarState) => { tab: SidebarTab; patch?: Partial<SidebarState> } | null
   /**
    * External-link target claim (v0.13.0+): when a GUI external-link click
-   * is taken over (the `browserInterceptLinks` master AND the URL's
-   * protocol flag — `browserInterceptHttp` / `browserInterceptHttps` —
-   * are on), the first registered tab whose `urlTarget(url)` returns true
-   * is opened with `openTab({ type, url, title: hostname })` — the URL is
-   * the whole payload (the tab reads it from `tab.path`). Registration
+   * is taken over, the first registered tab whose `urlTarget(url)` returns
+   * true is opened with `openTab({ type, url, title: hostname })` — the URL
+   * is the whole payload (the tab reads it from `tab.path`). Registration
    * order wins (first claim first served); a disabled tab type is skipped;
    * a throwing predicate is swallowed (console.error, the type is skipped).
-   * The built-in browser tab declares NO urlTarget — it stays the implicit
-   * fallback target, so plugins can never be shadowed by it. To host more
-   * than one URL at a time, mint per-URL ids through `createTab` (the
+   * A click no type claims is NOT taken over at all: it stays with whoever
+   * rendered the link (DSH 0.1.7's own chat view routes http(s) by the
+   * user's link-opening preference and falls back to a real browser tab).
+   * To host more than one URL at a time, mint per-URL ids through `createTab` (the
    * browser builtin's pattern); otherwise the id safety net focuses the
    * existing tab of the same type and the new URL is not applied.
    */
@@ -634,7 +633,7 @@ export function matchUrlTarget(tabs: readonly TabDescriptor[], url: URL): TabDes
  * The plugin version this service instance reports. Keep in lockstep with
  * `package.json`'s version — `tests/service.spec.ts` asserts the pair.
  */
-export const SIDEBAR_SERVICE_VERSION = '0.20.0'
+export const SIDEBAR_SERVICE_VERSION = '0.21.0-alpha.1'
 
 /**
  * Monotonic capability list consumers use to gate new API usage (features