[Task 5] Fix shell integration test isolation

[arittr]

Fix mock pollution in shell integration tests by using cache-busting dynamic imports.

Issue: Agent unit tests use mock.module() which creates persistent module mocks. Even after mock.restore(), Bun's module cache still serves the mocked version to subsequent imports.

Solution:

• Dynamically import shell module in beforeAll hook after calling mock.restore()
• Add timestamp query parameter to force fresh import and bypass module cache
• Rename test file to zzz-shell.integration.test.ts for consistent ordering

Result: All 14 failing tests now pass

• Before: 568 pass, 14 fail
• After: 582 pass, 0 fail

Tests verified: bun test completes successfully

[arittr]

This change is part of the following stack:

• spec: add agent standardization refactor specification [750f25] #69
  • refactor: standardize Codex agent execution pattern #70
    • [Task 3] Remove manual mode and add quiet flag #71
      • [Task 4] Update documentation and migration guide #72
        • [Task 5] Fix shell integration test isolation #73 ◀

_{Change managed by git-spice.}