Remove the retired awaiting-approval label from labels.json and its test #702

Summary

AGENTS.md now says awaiting-approval is retired (PR #700), but the label is still defined and still tested: .github/labels.json carries an awaiting-approval entry, and .github/scripts/sync-labels.test.mjs:23 still requires it in the required-names list. sync-labels.mjs therefore keeps creating it on the repository, so the "retired" sentence in the docs is untrue for the repository itself.

Change

• .github/labels.json: remove the awaiting-approval object. Every other entry stays exactly as it is — same names, same colours, same descriptions, same order (a full-file re-sort would hide the one deletion in the diff; keep it a one-hunk change).
• .github/scripts/sync-labels.test.mjs:23: drop 'awaiting-approval' from the required-names list. Keep the rest of the list, and do not relax the assertions around it (uniqueness, description length, colour shape).
• AGENTS.md keeps its sentence that the label is retired — that sentence is the record of why the entry is gone, so it is not a stale reference.
• Do not delete the label on GitHub by hand as part of this PR; sync-labels.mjs is what reconciles the repository, and a deleted entry is what makes the next sync stop creating it. (Its existing instances are already gone: no open issue or PR carries it.)

Acceptance

• grep for awaiting-approval: .github/labels.json no match; .github/scripts/sync-labels.test.mjs no match; AGENTS.md still matches in the retirement sentence. Nothing else in the tree matches.
• node --test .github/scripts/sync-labels.test.mjs green at the tip, and the label list still has 12 entries.
• The labels.json diff is a single deletion (one removed object, no reordering, no description edits) — the reviewer should be able to see it as one hunk.
• Red form: re-adding the entry to labels.json without touching the test makes the test's required-names assertion pass while a count/uniqueness assertion fails, or the reverse — either way the pair must be updated together; a PR that changes only one of the two files fails the suite.

Tier: .github/** → T3 (PASS at the tip, then the queue).