fix(cli): keep the commit-session trailer dedup working on git 2.39 #940

Observed

cli/tests/contexts/telemetry/domain/formats/commit-session-trailer.integration.test.ts:72 ("never doubles a trailer a prior run already wrote") runs the trailer-writing delegate twice and asserts the trailer token appears once.

On git 2.39.2 the test fails with AssertionError: expected 2 to be 1: the fixture message has no trailing newline, so git appends the trailer without a blank-line separator; the second pass then sees no trailer block and --if-exists doNothing appends again. The same delegate on a message ending in a newline dedupes correctly.

Reproduction

1. git --version -> 2.39.2 (or any version sharing this behavior).
2. cd cli && npx vitest run tests/contexts/telemetry/domain/formats/commit-session-trailer.integration.test.ts -> fails alone, deterministic, also under env -i.
3. The same test passes on newer git (CI's).

Impact

The suite reports one failure per run on machines with this git, masking real regressions in the file and eroding trust in the suite's signal. The test and its source are byte-identical to next, so the failure predates any recent change.

Suggested fix

End the fixture message with a newline, or have the delegate normalize the message before appending.

Standalone repro verified on the machine that reported it: message without trailing newline doubles the trailer; message with one dedupes.