feat(providers): restore ChatGPT Web via clean-room browser transport (#12239)

Restores ChatGPT Web on a clean-room browser transport, merged on the operator's explicit decision.

Worth stating precisely, because this touches a provenance decision: the PR does not revert #11754. It narrows RETIRED_COMMON_CHATGPT_WEB_PROVIDER_IDS to the single GPL-derived alias cgpt-web and registers chatgpt-web as a separate clean-room id. The old implementation stays retired and blocked; the retirement machinery, its error code and its 410 contract are untouched. All four retirement suites agree with that distinction and pass unchanged.

The 44 protected agent-instruction surfaces this PR touches (AGENTS.md, llm.txt and its 42 mirrors, README) were verified rather than trusted: masking digits and comparing the removed and added line sets gives 264 lines on each side, identical — every change is a provider-count substitution, with no sentence added, removed or reworded.

Reconciled on merge: clean against the tip, with the two chat chokepoints this PR grows (src/sse/handlers/chat.ts +40, open-sse/handlers/chatCore.ts +30) recorded in the file-size baseline under an annotation. Verified that exactly those two caps move and nothing else, so the #12411 ratchet holds. The rebaseline is carried on this branch rather than left in a validation worktree — the propagation mistake that put the 2026-09-02 merge waves base-red in #12434.

Verified: 504/504 across the PR's 44 test files plus all four chatgpt-web retirement suites, check:provider-consistency OK (272 REGISTRY entries, 355 canonical providers), check-file-size OK, and every changed TypeScript file parses.

Thanks @backryun — separating the clean-room id from the retired alias, instead of reopening the old one, is what made this reviewable.