Add interactive setup mode to devlog init

[gustmrg]

Extend devlog init with an interactive onboarding flow for first-time setup.

Context:

• Current init creates the base config but does not guide the user through setup
• DevLog has several useful defaults that are easier to configure once during onboarding
• An interactive mode would improve first-run UX without removing the ability to script initialization

Expected behavior:

• Add an interactive mode, preferably via devlog init --interactive
• Prompt the user for initial configuration values such as:
  • default project
  • default summary style
  • default language
  • whether AI summaries should be enabled
• If AI is enabled, prompt for:
  • provider
  • model
  • API key environment variable name
    Important behavior:
• Do not prompt for or persist the actual API key secret value in config
• Instead, store the environment variable name and print follow-up instructions for exporting the key in the shell
  Related improvements:
• Make init truly idempotent
• Create the full directory structure:
  • ~/.devlog/
  • ~/.devlog/entries/
  • ~/.devlog/summaries/
  • ~/.devlog/templates/
• Avoid overwriting existing config unless explicitly confirmed or forced

Implementation notes:

• Keep non-interactive devlog init available for automation
• Interactive mode should use existing config defaults as suggested values where appropriate
• At the end of setup, print a short “next steps” guide

Acceptance criteria:

• devlog init --interactive exists and appears in help output
• The user can complete a guided setup flow
• Chosen values are saved to ~/.devlog/config.json
• The full directory structure is created if missing
• Existing config is not overwritten silently
• The command prints clear next steps after initialization