Align devlog summary create --style behavior with README

[gustmrg]

Fix the mismatch between documented summary styles and current implementation.

Context:

• README.md presents --style as a summary formatting option
• Current implementation rejects --style unless --ai is also set
• Saved summaries are currently always marked as concise

Expected behavior:

• --style should work according to the product decision:
  • either for both AI and non-AI summaries
  • or only for AI summaries, with README updated to match
• The saved summary metadata should reflect the actual chosen style

Implementation notes:

• Decide whether styles like concise, detailed, formal, and impersonal should be available without AI
• If non-AI styles are supported, implement formatting differences in local summary generation
• If AI-only is the intended behavior, adjust docs and help text instead

Acceptance criteria:

• --style behavior matches the documented product behavior
• Saved summaries record the real style used
• Help text and README are consistent with implementation