test: stop Windows developer tooling from leaking into setup, tar, and audit tests

On a Windows machine with Claude Code installed natively, Git for Windows,
and the ECC plugin installed, `node tests/run-all.js` on main fails 44 tests
in 7 files. Linux CI never sees them. Three of the four causes are test
isolation gaps, fixed here (43 tests):

- claude-plugin-setup, claude-scope-migration, setup (38): fixtures put a
  fake claude.cmd first on PATH, but on Windows spawning a bare `claude`
  resolves only .com/.exe, so a native ~/.local/bin/claude.exe ran instead
  of the fake. Fixture PATHs now drop entries holding claude.exe/claude.com.
- ecc-universal-bin, eval-harness-package (4): tar received an absolute
  C:\ archive path, which Git Bash's GNU tar reads as a host:path remote
  spec ("Cannot connect to C: resolve failed"). Extract from inside the
  destination with a relative, forward-slash archive name, which both GNU
  tar and bsdtar accept.
- harness-audit (1): the cache-layout test overrode HOME only, while
  findPluginInstall also reads USERPROFILE and os.homedir(), so the real
  installed plugin won over the fixture. Override USERPROFILE too.

The two helpers live in tests/lib/helpers/windows-test-env.js with a
platform-independent test, so Linux CI holds both invariants too.

The 44th failure, plugin-hook-bootstrap's no-bash test, is a production
bug rather than a test leak: System32\bash.exe launches an installed WSL
distro, which passes the bash probe. It gets a separate fix.

tests/scripts/setup.test.js

@@ -5,6 +5,7 @@ const fs = require('fs');
 const os = require('os');
 const path = require('path');
 const { spawnSync } = require('child_process');
+const { withoutShadowingClaude } = require('../lib/helpers/windows-test-env');
 const repoRoot = path.join(__dirname, '..', '..');
 const setupScript = path.join(repoRoot, 'scripts', 'setup.js');
 const eccScript = path.join(repoRoot, 'scripts', 'ecc.js');
@@ -62,7 +63,7 @@ function runSetup(fixture, args, options = {}) {
     HOME: fixture.homeDir,
     USERPROFILE: fixture.homeDir,
     CLAUDE_CONFIG_DIR: fixture.configDir,
-    PATH: options.path || `${fixture.binDir}${path.delimiter}${process.env.PATH || ''}`,
+    PATH: options.path || `${fixture.binDir}${path.delimiter}${withoutShadowingClaude(process.env.PATH)}`,
     ECC_TEST_CLAUDE_STATE: fixture.statePath,
     ECC_TEST_CLAUDE_CALLS: fixture.callsPath,
     ...options.env,
@@ -147,7 +148,7 @@ function runInteractiveEccSetup(fixture, options = {}) {
       HOME: fixture.homeDir,
       USERPROFILE: fixture.homeDir,
       CLAUDE_CONFIG_DIR: fixture.configDir,
-      PATH: `${fixture.binDir}${path.delimiter}${process.env.PATH || ''}`,
+      PATH: `${fixture.binDir}${path.delimiter}${withoutShadowingClaude(process.env.PATH)}`,
       ECC_TEST_CLAUDE_STATE: fixture.statePath,
       ECC_TEST_CLAUDE_CALLS: fixture.callsPath,
     },