fix: Windows-native CI findings on latest master

Local run of check:ci:windows-complete (the windows-native gate) on
latest master surfaced five Windows-only failures, all unreachable by
current CI because the native windows job is disabled and the wine gate
only covers build+site.

- install-lefthook/translation-pairing-merge specs junctioned the real
  scripts/ and tsx package into fixtures; Windows recursive deletion
  (Node rmSync and git worktree remove) follows MOUNT_POINT junctions and
  deleted the repository's own directories mid-run. Fixtures now unlink
  their reparse points before any recursive removal (shared helper in
  scripts/test-fixture-cleanup.ts).
- workflow-workerthread spawned its worker with an empty env; on Windows
  os.tmpdir() then degrades to the literal relative path undefined\temp,
  so tsx wrote its transform cache into a cwd-relative undefined/
  directory inside the repo. The worker env now injects the host temp
  path on win32 (workerSpawnEnv, platform-parameterized and unit-tested
  on both arms).
- workspace-context spec did not stub USERPROFILE (win32 homedir) or a
  set DSH_HOME, leaking the developer machine's real ~/.dsh/AGENTS.md
  into discovery.
- ui-trajectory client-bundle spec mounted the built artifact without the
  remote/settingsScope provides the locale plugin needs, so the plugin
  never activated and no view registered.
- subagent temp-fixture cleanup lacked the maxRetries Windows handle
  release needs under load (EPERM); added retries to the three affected
  specs and the fixture-cleanup helper.

scripts/install-lefthook.spec.ts

@@ -16,6 +16,7 @@ import { tmpdir } from 'node:os'
 import { dirname, isAbsolute, join, resolve } from 'node:path'
 import { fileURLToPath } from 'node:url'
 import { afterEach, describe, expect, it } from 'vitest'
+import { removeFixtureSafely, unlinkFixtureLinks } from './test-fixture-cleanup.ts'
 
 const installer = fileURLToPath(new URL('./install-lefthook.mjs', import.meta.url))
 const pairingMergeDriver = 'scripts/merge-translation-pairing-driver.sh %O %A %B %P'
@@ -40,7 +41,7 @@ interface CommandResult {
 }
 
 afterEach(() => {
-  for (const fixture of fixtures.splice(0)) rmSync(fixture, { recursive: true, force: true })
+  for (const fixture of fixtures.splice(0)) removeFixtureSafely(fixture)
 })
 
 function commandResult(command: string, args: string[], cwd: string, env: NodeJS.ProcessEnv): CommandResult {
@@ -282,6 +283,10 @@ describe('worktree-local Lefthook installer', { timeout: 15_000 }, () => {
     expect(gitResult(fixture, fixture.main, ['config', '--file', commonConfig, '--get', 'core.bare']).status).toBe(1)
 
     const mainHookBeforeRemoval = readFileSync(join(mainHooks, 'pre-commit'), 'utf8')
+    // Windows Git follows the fixture's MOUNT_POINT junctions into their real
+    // targets while removing a worktree; unlink them first so the removal
+    // cannot delete the repository's scripts/ or tsx package.
+    unlinkFixtureLinks(fixture.linked)
     git(fixture, fixture.main, ['worktree', 'remove', '--force', fixture.linked])
     expect(readFileSync(join(mainHooks, 'pre-commit'), 'utf8')).toBe(mainHookBeforeRemoval)
     expect(readFileSync(legacyHook, 'utf8')).toBe('#!/bin/sh\n# legacy hook\n')