fix: run remi-diary summary via codex exec with safe prompt piping

[lsj5031]

Summary

• switch scripts/remi-diary.sh summary generation from pi prompt argument passing to codex exec
• feed prompt via stdin and capture final summary via --output-last-message temp file
• add robust fallback when summary command is missing/fails so diary generation still succeeds
• update README warning text to reflect configurable summary provider

Why

• avoids argument length overflow from passing very large prompts as command args
• removes hard dependency on pi API key for diary summary generation path
• preserves daily diary output even when AI summary step fails

Verification

• scripts/remi-diary.sh --date yesterday (in this environment codex is missing; script now degrades gracefully)
• DIARY_SUMMARY_CMD='nonexistent-cmd' scripts/remi-diary.sh --date yesterday (confirmed fallback summary + explicit error in markdown)