feat(agent): ban regex-scanning source code in tests

Add a new AGENTS.md antipattern section: tests should NEVER read source
code and regex against it!

AGENTS.md

@@ -1354,3 +1354,58 @@ not the specific names.
 
 Reviewers should reject new change-detector tests; authors should convert
 them into invariants before re-requesting review.
+
+### Never read source code in tests
+
+A test that reads a source file's text is testing *the shape of the
+source code*, not its behavior. This is a hard antipattern, banned outright.
+Any test that reads a .py, .ts, .tsx, etc., file is suspect.
+
+**Why it's actively harmful, not just weak:**
+
+- It passes when the implementation is subtly broken (the regex matches a
+  call site that exists but is wired wrong) and fails when a correct
+  refactor changes formatting, variable names, or control flow with
+  identical runtime behavior. Both directions of failure are wrong.
+- It can't be run against a built/bundled/minified artifact, so it silently
+  stops testing anything the moment code moves, gets renamed, or a
+  dependency reformats it.
+- It actively blocks refactors: reviewers see "keeps a pattern intact" tests
+  fail during pure structural cleanup with no behavior change, and either
+  hand-wave the failure (dangerous) or waste time updating regexes that add
+  nothing (waste).
+- It gives false confidence. a green suite full of source-regex tests
+  looks like coverage but has never once executed the code path it claims
+  to guard.
+
+**Do not write:**
+
+```ts
+const source = fs.readFileSync(path.join(__dirname, 'main.ts'), 'utf8')
+
+test('backend spawn hides the Windows console', () => {
+  assert.match(source, /spawn\(\s*backend\.command,\s*backend\.args[\s\S]{0,300}hiddenWindowsChildOptions/)
+})
+```
+
+**Do write — extract the logic into a small pure/DI-testable function and
+call it for real:**
+
+```ts
+// backend-spawn.ts
+export function hiddenWindowsChildOptions(options: SpawnOptionsLike = {}, isWindows = process.platform === 'win32') {
+  if (!isWindows || 'windowsHide' in options) return options
+  return { ...options, windowsHide: true }
+}
+
+// backend-spawn.test.ts
+test('windowsHide defaults to true on Windows, is left alone elsewhere', () => {
+  assert.equal(hiddenWindowsChildOptions({}, true).windowsHide, true)
+  assert.equal(hiddenWindowsChildOptions({}, false).windowsHide, undefined)
+  assert.equal(hiddenWindowsChildOptions({ windowsHide: false }, true).windowsHide, false)
+})
+```
+
+If the logic lives inline in a god-file (`main.ts`, `cli.py`,
+`gateway/run.py`) and extracting it feels disruptive: that's the actual
+signal to do the extraction, not to regex around it.