chore: remove vestigial CHANGELOG.md

[shaanmajid]

What problem are you trying to solve?

CHANGELOG.md and RELEASE-NOTES.md both exist but only RELEASE-NOTES.md is maintained — CHANGELOG.md contains only v5.0.5 entries, all of which are fully mirrored in RELEASE-NOTES.md. Two release docs with one stale creates drift risk and reviewer confusion.

What does this PR change?

Deletes CHANGELOG.md. No code, no behavior, no skill content touched.

Is this change appropriate for the core library?

Yes — it removes an artifact from the repo itself.

What alternatives did you consider?

Porting the one extra detail (the (Reverts 5e51c3e) commit-SHA breadcrumb) into RELEASE-NOTES.md first. Skipped because the revert is already self-evident from the v5.0.5 changed-behavior bullet and from git log.

Does this PR contain multiple unrelated changes?

No — single file deletion.

Existing PRs

• I have reviewed all open AND closed PRs for duplicates or prior art
• Related PRs: none found

Environment tested

Harness	Harness version	Model	Model version/ID
N/A — documentation-only change, no runtime behavior to exercise

Evaluation

N/A — this is a file deletion of an unmaintained doc. Nothing to eval.

Rigor

• If this is a skills change: I used superpowers:writing-skills and completed adversarial pressure testing — N/A, not a skills change
• This change was tested adversarially, not just on the happy path — N/A, file deletion
• I did not modify carefully-tuned content (Red Flags table, rationalizations, "human partner" language) without extensive evals showing the change is an improvement

Human review

• A human has reviewed the COMPLETE proposed diff before submission

[obra]

THANK YOU! I was just complaining about this this morning.

[shaanmajid]

Thank you @obra :^)