Implement devlog config set, get, and list

[gustmrg]

Implement the devlog config command documented in README.md.

Context:

• README.md documents:
  • devlog config set <key> <value>
  • devlog config get <key>
  • devlog config list
• The current cmd/config.go command is only a stub and does not implement any subcommands
• The project already uses Viper and writes initial config in internal/store/store.go

Expected behavior:

• devlog config set <key> <value> updates configuration and persists it to ~/.devlog/config.json
• devlog config get <key> prints the current value for a supported key
• devlog config list prints the current configuration in a readable format

Supported keys should include the documented config values, including:

• defaults.project
• defaults.style
• defaults.language
• llm.enabled
• llm.provider
• llm.model
• llm.apiKeyEnvVar

Implementation notes:

• Align CLI key names with the actual config structure used in code and docs
• Validate unknown keys and invalid value types with clear error messages
• Support boolean parsing for keys like llm.enabled
• Ensure changes are written back safely without corrupting the existing config file

Acceptance criteria:

• devlog config set, get, and list exist and appear in help output
• Config values persist correctly to ~/.devlog/config.json
• Unknown keys return a clear error
• Boolean and string values are parsed correctly
• Behavior matches the README documentation