Kickoff: Implement Configuration Foundation

[duizendstra]

As a developer of ContextVibes CLI, I want to extend internal/config/config.go and related config handling so that new state (projectState.*) and AI collaboration preferences (ai.collaborationPreferences.*) can be robustly loaded, saved, and merged with defaults in .contextvibes.yaml.

This enables the kickoff command to manage its state and user preferences.

Related Epic: #EPIC_ISSUE_NUMBER

Acceptance Criteria:

• internal/config/config.go:
  • Config struct includes ProjectState and AISettings (with AICollaborationPreferences).
  • GetDefaultConfig() initializes these new structs/fields with sensible defaults.
  • MergeWithDefaults() correctly merges these new nested structs, prioritizing user-defined values.
  • UpdateAndSaveConfig() successfully saves the updated config structure to .contextvibes.yaml.
• Unit tests in internal/config/config_test.go cover:
  • Default values for new fields.
  • Correct merging of new fields.
  • Successful saving and loading of new fields.
• An initial .contextvibes.yaml (or update existing sample) is created in the contextvibes/cli repo root for dogfooding/testing, including placeholders/examples for the new sections.