test(strategy): add red tests for squash-inheritance edge cases

The squash half of the adversarial review of #2574 (split from 4a5df627d0; the redo half moves with the reset PR). Each test asserts the linkage a user would want.

Failing (2):

• PostCommit condenses into an inherited checkpoint that is not in the local store: a declined -m squash of a teammate branch.
• Squash with an edited file strips git's seeded trailer (regression from main).

Passing guard (1): user-copied trailer not duplicated.