proposal: add change manager - extract + new functionality

Slice 2 of the artifact tracker POC. Creates ChangeManager module that:

**Extracts existing functionality:**
- `listChanges()` from ListCommand + ChangeCommand.getActiveChanges()
- `changeExists()` from inline fs.access() checks
- `getChangePath()` from inline path.join() calls
- `isInitialized()` from ListCommand directory check

**Adds new functionality:**
- `createChange(name, description?)` - create change directory + README
- `validateName(name)` - enforce kebab-case naming

**Refactors CLI commands to be thin wrappers:**
- ListCommand delegates to ChangeManager
- ChangeCommand delegates to ChangeManager

Also updates docs/artifact_poc.md to reflect XDG decisions from Slice 1.

Author: TabishB