agent: ensure today's daily note file exists before LLM read_file

[seanly]

• Add EnsureTodayFile() to create memory/YYYYMM/YYYYMMDD.md with date header if missing
• Call it from GetMemoryContext() so path advertised in system prompt always exists
• Avoids read_file tool failure when agent (or cron) tries to read today's note

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• 🐞 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 📖 Documentation update
• ⚡ Code refactoring (no functional changes, no api changes)

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[xiaket]

How is this different from AppendToday?

[nikolasdehor]

Good idea — proactively creating the daily note file avoids a common LLM tool failure. Two concerns:

1. TOCTOU race in EnsureTodayFile: The function checks os.Stat then writes with os.WriteFile. If two goroutines (e.g., concurrent cron jobs) call GetMemoryContext simultaneously, both can pass the Stat check and both write the file. This is not catastrophic (the second write just overwrites with the same header), but use os.OpenFile with O_CREATE|O_EXCL to atomically create-if-not-exists:

f, err := os.OpenFile(todayFile, os.O_CREATE|os.O_EXCL|os.O_WRONLY, 0o644)
if err \!= nil {
    return // already exists or other error
}
f.Write([]byte(header))
f.Close()

2. Silently swallowing errors: Both MkdirAll and WriteFile errors are silently ignored (return / _ =). If the file cannot be created (e.g., permissions issue on workspace dir), the LLM will still hit the read_file failure downstream. At minimum, consider logging a warning so the user knows why daily notes are not working.

Otherwise a clean, targeted fix.