chore: Remove the decline_responses_websockets translation after a deprecation release #1625

What

v0.9.4 shipped listener.decline_responses_websockets, a boolean. #1467 replaces it with listener.responses_websockets: parse | decline | forward. To keep v0.9.4 configs working, #1467 still reads the old key when the new one is unset (true → decline, false → forward), logs a WARN naming the new key, and refuses a config that sets both.

This issue tracks removing that translation.

When

Not before both of these:

• At least one release has shipped the warning. A user who set the old key should have had a release that told them to change it before one that refuses it.
• A release the project is willing to label a breaking change. Before 1.0, semver allows a breaking change in any release, and this repo has done it: Breaking change: Upgrade the cpex plugin to CPEX 0.3.1 #1429 shipped in v0.9.2, a patch release, listed under "Breaking changes" in its notes. So this does not have to wait for 1.0, but it needs a deliberate release-notes entry, not a silent removal.

What removal means

Do not delete the field outright. If the field were simply deleted, the YAML would ignore the key, and a config that set decline_responses_websockets: false would quietly get the default, parse, with no word said. Replace the translation with a startup refusal that names the mode the old value maps to. That refusal is what #1467 did before v0.9.4 shipped the key, and it is in that PR's history.

• core/config/config.go: DeclineResponsesWebSockets and the fallback in ResponsesWebSocketsMode.
• core/config/validate.go: the WARN and the both-set refusal become a single refusal.
• TestLoad_DeprecatedDeclineKeyStillApplies becomes a refusal test.
• Docs: docs/agents/codex.md, docs/framework-architecture.md and CLAUDE.md describe the old key; drop those lines.

PR title prefix: Breaking change:, so the release notes say so.