chore: drop linear/issue ticket references from source comments (#3159)

54 references across 18 files (PUT-1010, PUT-1014, PUT-1019, PUT-1021,
PUT-1022, PUT-1023, PUT-1024 + sub-tags AUTH-2/4/5, SDK-1, PJS-1/2,
GUI-1/2, ROLLOUT-1) removed from inline comments, doc comments,
test describe blocks, and SQL migration headers. The substantive
explanations stay; only the ticket pointers go.

No behavior change. Full backend test suite: 2172 passed / 16 skipped.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>