refactor(code-runtime): address seam review — drop worker aliases, tighten dunder

- Worker consumes PORTABLE_RESERVED_WORDS / RESERVED_ERROR_MEMBERS by
  their seam names directly, dropping the local re-alias (symmetry with
  the other two imported constants).
- Split the reserved-vs-duplicate diagnostics: a backend-owned global now
  reports "reserved binding global", not the misleading "duplicate".
- DUNDER_MEMBER uses `__.+__` so a bare `__` (empty middle, not a real
  CPython dunder) is not matched; add coverage.
- Worker misuse tests add `a$b` (second-char `$`) and `lambda` (Python
  keyword) so the identifier narrowing and reserved-word adoption are
  each pinned directly, not only transitively.
- Clarify the seam JSDoc (dunder-vs-explicit-set wording, Python backend
  is a later stack PR) and record in the Agent Note the obligation to
  widen RESERVED_BINDING_GLOBALS when the bootstrap seeds more globals.

packages/code-runtime/code-runtime/tests/reserved.spec.ts

@@ -35,6 +35,9 @@ describe('seam-owned portable identifier exclusions', () => {
     expect(DUNDER_MEMBER.test('_private')).toBe(false)
     expect(DUNDER_MEMBER.test('name')).toBe(false)
     expect(DUNDER_MEMBER.test('__mid')).toBe(false)
+    // `__` has an empty middle — not a real CPython dunder, so not matched.
+    expect(DUNDER_MEMBER.test('__')).toBe(false)
+    expect(DUNDER_MEMBER.test('____')).toBe(true)
   })
 
   it('PORTABLE_RESERVED_WORDS is the union of ECMAScript and Python reserved words', () => {