Warn when CODEX_HOME is full; avoid silent loss of session history

Feature request / bug report

Codex should detect when the CODEX_HOME area is full and warn the user instead of continuing silently and losing session history.

Problem

When the CODEX_HOME filesystem fills up, Codex appears to keep working but does not record new session data. This leads to:

• loss of resume capability for new sessions
• existing sessions missing new conversation history
• no visible warning/error to indicate persistence is failing

Expected behavior

• Detect low/zero disk space (or write failures) in CODEX_HOME
• Surface a clear warning/error when session persistence fails
• Avoid silent data loss; optionally pause or degrade gracefully

Acceptance criteria

• Codex detects and reports write failures to CODEX_HOME
• User receives a clear notification with remediation guidance
• Session history is not silently dropped