Fix all plan review issues, save implementation plan

Plan fixes:
- detect_platform raises FlowError not RuntimeError
- TargetConfig lives in core/config.py only (remote domain imports it)
- plan_link handles source changes (remove_link + create_link)
- resolve_package_targets skips local files when mount_path is root
- LinkedState.from_dict guards on version mismatch
- Added missing test for parse_module_ref with absent ref
- Task 12 now has full tests and serialization format
- Task 13 uses spec signatures as truth, old code as reference
- Task 15 includes describe() examples and tests
- Task 24 has detailed test cases for ProjectService
- Note that conflicts.py is intentionally merged into planning.py
- Spec Section 12 example fixed to include filesystem_check arg

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/superpowers/specs/2026-03-16-flow-architecture-redesign.md

@@ -1225,7 +1225,7 @@ def test_plan_link_creates_ops_for_new_targets():
     desired = [LinkTarget(source=Path("/a"), target=Path("/home/x/.zshrc"),
                           package="_shared/zsh", from_module=False, needs_sudo=False)]
     current = LinkedState(links={})
-    plan = plan_link(desired, current)
+    plan = plan_link(desired, current, filesystem_check=lambda p: None)
     assert len(plan.operations) == 1
     assert plan.operations[0].type == "create_link"