Add --date parameter to 'nost n' command for backdating notes

[amy-shinsen-openclaw]

Description

This PR adds support for specifying a custom date when creating notes using the nost n command.

Motivation

Previously, the nost n command would always create notes for today's date. This feature allows users to create notes for any past date, which is useful for:

• Catching up on missed days
• Backdating entries
• Testing the system with specific dates

Changes

• Modified src/commands/new.rs to parse the --date parameter
• Updated src/dates/get.rs to accept and parse custom dates in YYYY-MM-DD format
• Modified src/files/create.rs to accept and pass the date parameter
• Updated all callers of create_file() to pass the date parameter

Usage

# Create a note for today (existing behavior)
nost n

# Create a note for a specific date
nost n --date "2026-07-20"

# Create a note with title for a specific date
nost n "My title" --date "2026-07-20"

Testing

• ✅ Code compiles without errors
• ✅ Manual testing confirms nost n --date "2026-07-20" creates note for July 20th
• ✅ File is created at correct path with correct metadata