chore: release v0.3.0

[bkataru]

Summary

Version bump to 0.3.0 for the Phase 2 release.

Changes

• Update workspace.package.version from 0.1.0 to 0.3.0
• Update all crate versions from 0.2.0 to 0.3.0
• Update internal dependency version references
• Add CHANGELOG.md with v0.3.0 release notes
• Update documentation for new features

New Features in v0.3.0

• SkillExecutor trait for pluggable execution strategies
• ExecutionContext for managing inputs/outputs between steps
• ExecutionHooks trait with TracingHooks and CompositeHooks
• DefaultSkillExecutor implementation using Transport
• Session management with turn counting and limits
• SessionManager for async file-based persistence
• Session filtering and querying capabilities
• thulp-skill-files crate for SKILL.md file parsing
• YAML frontmatter parsing for skill files
• Preprocessor for variable substitution
• Multi-directory skill discovery with scope priority
• Timeout and retry support for skill execution
• TimeoutConfig, RetryConfig, ExecutionConfig types

Post-Merge Actions

After merging, create the release:
```bash
git checkout master && git pull
git tag -a v0.3.0 -m "thulp v0.3.0 - Phase 2 Complete"
git push origin v0.3.0
gh release create v0.3.0 --title "thulp v0.3.0 - Phase 2 Complete" --notes-file CHANGELOG.md
```