🤖 refactor: simplify fork tests to 5 orthogonal cases

Reduced test suite from 10 to 5 focused tests while preserving coverage:

Before:
- 10 tests (20 total including local + SSH variants)
- Multiple redundant happy-path tests
- Tests scattered across 4 describe blocks

After:
- 5 orthogonal tests (10 total including local + SSH variants)
- Each test covers a distinct dimension:
  1. Validation (invalid workspace names)
  2. Runtime config preservation + workspace usability
  3. Chat history copying
  4. Uncommitted file preservation (SSH only)
  5. Init state management (can't fork during init, tools blocked until init)

Removed redundant tests:
- "should fork workspace successfully" (covered by #2)
- "should fork workspace and send message" (covered by #2)
- "should run init hook when forking" (covered by #5)
- "should preserve git state" (implicitly covered - workspace functions)
- "should fail to fork while initializing" (merged into #5)

All tests still pass (10 total: 5 tests × 2 runtimes).

Author: ammar-agent