Make devlog init idempotent and create the full data directory structure

[gustmrg]

Improve devlog init so it is safe to run multiple times and fully prepares the local DevLog workspace.

Context:

• README.md says devlog init is safe to run multiple times and will not overwrite existing data
• Current implementation uses viper.SafeWriteConfig(), which can fail when the config already exists
• Current implementation creates only the base ~/.devlog/ directory and writes config, but does not ensure subdirectories exist

Expected behavior:

• devlog init can be run repeatedly without failing in normal already-initialized cases
• Existing config should not be overwritten
• Missing directories should be created automatically
• The command should report what was created versus what already existed
• Directories that should be ensured:
  • ~/.devlog/
  • ~/.devlog/entries/
  • ~/.devlog/summaries/
  • ~/.devlog/templates/

Implementation notes:

• Preserve existing user config if config.json is already present
• Create config only when it does not already exist
• If some directories are missing, create only the missing ones
• Return real errors only for actual failure cases such as permission issues or invalid paths
• Consider printing a friendly summary such as:
  • config already exists
  • created entries/
  • created summaries/

Acceptance criteria:

• Running devlog init on a fresh machine creates config and required directories
• Running devlog init again succeeds without overwriting existing config
• If one or more subdirectories are missing, they are recreated
• The command output clearly indicates what happened
• Behavior matches the README claim that init is safe to run multiple times