Refuse ambiguous changelog comparison histories

[devantler]

🤖 Generated by the Agentic Engineer

Problem

A changelog check can silently choose one of several merge bases and miss a version change that requires release notes.

Proposal

Require one complete, unambiguous comparison base and complete history before either changelog mode.

Acceptance criteria

Real crisscross and incomplete histories refuse before writes or a clean report; a unique complete merge base continues to validate the branch's actual changes.

Reproduced offline against freshly fetched main using synthetic fixtures. No inspected source was executed and no provider experiment was launched. Size: S.

Part of #101.

[devantler]

🤖 Generated by the Agentic Engineer

Delivered in merged PR #486 at reviewed head d324854. The regression and actual user-path checks passed before merge; merged source was read back.