docs: sync dedupe and conflicts contracts

[CoderDeltaLAN]

Summary:

• Documents the post-v0.3.0 dedupe and conflicts commands in README, changelog, exit-code contract, and output contract.
• Keeps release/PyPI wording honest: these commands are on current main and not claimed as published package behavior until a future verified release.

Validation:

• Local CLI smoke for dedupe/conflicts passed.
• Full local checks passed.

Risk:

• Documentation-only change.

Rollback:

• Revert this PR.