Implement weekly summary generation for devlog summary create --week

[gustmrg]

Add support for generating weekly summaries with devlog summary create --week.

Context:

• README.md documents weekly summary generation
• Current implementation only loads one daily entry file

Expected behavior:

• Collect entries across the current week, or a target week if extended later
• Group entries into a single summary
• Save the generated summary under ~/.devlog/summaries/
• Display an appropriate success message and file target

Implementation notes:

• Decide on filename convention for weekly summaries, for example:
  • 2026-W16.md
  • 2026-04-13_to_2026-04-19.md
• Ensure summary show and summary list can work cleanly with weekly summaries if needed
• Validate that --week is not used together with incompatible date flags unless explicitly supported

Acceptance criteria:

• devlog summary create --week exists and works
• The command aggregates entries across the week
• The output is saved predictably
• Empty-week behavior is clear and user-friendly