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/translation-pairing-merge.spec.ts

@@ -1,7 +1,14 @@
 /** Integration coverage for automatic and explicit pairing-record conflict resolution. */
 
 import { execFileSync, spawnSync } from 'node:child_process'
-import { chmodSync, mkdtempSync, mkdirSync, readFileSync, rmSync, symlinkSync, writeFileSync } from 'node:fs'
+import {
+  chmodSync,
+  mkdtempSync,
+  mkdirSync,
+  readFileSync,
+  symlinkSync,
+  writeFileSync,
+} from 'node:fs'
 import { tmpdir } from 'node:os'
 import { delimiter, dirname, join } from 'node:path'
 import { fileURLToPath } from 'node:url'
@@ -15,6 +22,7 @@ import {
   renderTranslationPairingRecord,
   translationPairPaths,
 } from './translation-pairing-record.ts'
+import { removeFixtureSafely } from './test-fixture-cleanup.ts'
 
 const driver = fileURLToPath(new URL('./merge-translation-pairing.ts', import.meta.url))
 const driverLauncher = fileURLToPath(new URL('./merge-translation-pairing-driver.sh', import.meta.url))
@@ -28,7 +36,7 @@ interface Fixture {
 }
 
 afterEach(() => {
-  for (const fixture of fixtures.splice(0)) rmSync(fixture, { recursive: true, force: true })
+  for (const fixture of fixtures.splice(0)) removeFixtureSafely(fixture)
 })
 
 function git(fixture: Fixture, args: string[]): string {