fix(browser): fail early disconnected sessions without conversation

Mark browser sessions as failed when Chrome disconnects before Oracle can prove a recoverable ChatGPT conversation exists, while preserving reattach for submitted prompts, live Chrome sessions, project URLs, legacy ChatGPT URLs, and Deep Research placeholders.

Proof:
- pnpm vitest run tests/browser/reattachability.test.ts tests/cli/sessionRunner.test.ts tests/browser/reattach.e2e.test.ts tests/browser/index.test.ts tests/browser/promptComposer.test.ts tests/browser/modelSelection.test.ts tests/browser/thinkingTime.test.ts
- pnpm run check
- pnpm run build
- codex-review clean: no accepted/actionable findings reported
- GitHub CI passed on 306749f0

Live browser smoke was attempted, but local ChatGPT auth was unavailable: ChatGPT session was not detected and the run stopped at the login page.