test(python): pin all four enum sets, and the git_branch actions in use

[CMGS]

Stacked on #81. Closes the Python half of a wire-corpus drift gap; the Rust half (Response encode paths are never fixture-verified) is left for its own change.

The gap

protocol/wire/fixtures/v1/enums.json is the shared anti-drift corpus. Go (TestEnumValueSetsMatchFixture) and Rust (enum_value_sets_match_fixture) both pin all four sets. Python pinned two: error_kind and event_kind.

Of the two missing, git_branch_action is load-bearing — sandbox.py:191,194,197,200 hardcodes "list", "checkout", "create", "delete" straight into the request. A rename on the silkd side would have reached users as a runtime rejection with nothing in the suite failing first.

What is added

• test_enum_value_sets_match_corpus now covers file_kind and git_branch_action alongside the existing two.
• test_git_branch_actions_come_from_the_corpus drives the four real verbs through a recording conn and asserts the actions they emit are exactly the corpus set — so the literals stay tied to the contract, not just the constant list.

Validation

• pytest 152 passed, ruff check clean
• mutation-tested: renaming action="checkout" to "switch" in sandbox.py fails the new test

[CMGS]

Squash-merged into main as a4c1c7d (the branch was stacked on #81, so only its own commit was cherry-picked — merging the branch would have dragged #81 in). pytest 152 passed and ruff clean on main.