[P2] Update OpenClaw skill with formatting and date guidelines

[KalebCole]

Problem

The OpenClaw skill (~/.openclaw/workspace/skills/partiful/SKILL.md) needs to document:

1. No markdown in descriptions - Partiful renders plain text only
2. Always include full year in dates - e.g., 2026-03-15 8am not Mar 15 8am
3. Timezone handling - Default is America/Los_Angeles but should be explicit

Skill should instruct:

• Use plain text formatting (no **, no -, use emoji for visual breaks)
• Always use ISO-ish dates with year: 2026-03-15 8am
• Verify event date after creation
• Double-check times (especially AM vs PM for early morning events)

Acceptance Criteria

• SKILL.md updated with formatting guidelines
• SKILL.md updated with date format requirements
• Examples in skill show correct patterns