test(ce-explain): guard predict-then-reveal parity across SKILL.md and check-in reference

[LukeTheoJohnson]

Closes #1057

Summary

Implements the parity guard requested in #1057. skills/ce-explain/SKILL.md Phase 3 and references/check-in.md both deliberately carry the predict-then-reveal protocol, but nothing stopped the two separate editable copies drifting apart silently: the routing test asserted only two substrings, in SKILL.md alone.

Fix

Extends tests/skills/ce-explain-routing.test.ts with a parity block asserting BOTH files carry the three invariants named in the issue:

• the prediction question (what does this change do, and why was it made)
• the turn-end rule ("end the turn")
• the never-same-message rule (no explanation in the same message as the prediction prompt)

The assertions are structural regexes. The two copies already word the protocol slightly differently, and future wording improvements stay possible as long as each invariant survives in both files. Failure messages name the sibling copy so whoever trips the guard knows the duplication is by design and where the other copy lives.

Verification

• Red probe: replacing "End the turn." with "Continue." in check-in.md fails exactly one test, naming the file and the missing invariant; restoring the line returns the suite to green. 11/11 tests in the file pass on this branch (5 existing + 6 new).
• Full bun test baseline diff against main on this machine: zero new failures.