Implement devlog summary list with date filtering

[gustmrg]

Add the missing implementation for devlog summary list.

Context:

• The command exists in cmd/summary/list.go but its handler is empty
• README.md documents listing generated summaries from ~/.devlog/summaries/
• Filtering by date is important for this command

Expected behavior:

• List generated summaries found in ~/.devlog/summaries/
• Support filtering by:
  • --date <YYYY-MM-DD> for a specific summary date
  • --from <YYYY-MM-DD> and --to <YYYY-MM-DD> for date ranges
  • --week, -w for current week
  • --month, -m for current month
• Show a clear empty state when no matching summaries are found

Implementation notes:

• The README currently documents --week, --month, --from, and --to
• Add --date as a first-class filter so users can target one exact summary directly from list output
• Validate conflicting filters and invalid date formats
• Sort output consistently, ideally newest first

Acceptance criteria:

• devlog summary list returns stored summaries
• devlog summary list --date 2026-04-13 filters to one date
• devlog summary list --from ... --to ... filters by range
• devlog summary list --week and --month work
• Invalid or conflicting filter combinations return clear errors