[codex] fix Web transcript layout and refresh history

[linjiaye929]

What changed

• keep the composer pinned inside the viewport while long Agent answers scroll in the transcript pane
• retain a bounded history of the latest 20 in-process runs and restore the full transcript after browser refresh
• protect prompt/answer history behind the existing process-random request token while keeping lifecycle events prompt-free
• reconnect SSE from the latest observed sequence instead of replaying duplicate activity
• enforce HTML hidden states and bump the pre-release version to 0.18.0a1

Root cause

The conversation grid item kept its automatic minimum block size, so a long response expanded the entire column and clipped the composer below the workbench. Completed runs were not discoverable from bootstrap, and browser state existed only in JavaScript memory. Native EventSource reconnects also reused the initial after=0 URL.

Validation

• TDD regression tests observed four expected failures before implementation
• py -3.13 -m uv run pytest --cov -q: 1224 passed, 13 platform/privilege skips, 88.50% coverage
• Ruff format/check and strict Pyright passed
• real-browser long-answer and two-run refresh checks passed at 1280x720 and 390x844
• composer bottom matched viewport bottom, transcript remained independently scrollable, all four messages restored, and browser console reported no errors