Add CHANGELOG.md and adopt semantic versioning

[dhilgaertner]

Parent Issue

Part of #52 — Open Source Readiness & Launch Strategy

Overview

Establish a versioning strategy and changelog to track releases. Currently version is hardcoded to 0.1.0 in scripts/bundle.sh.

Scope

• Create CHANGELOG.md using Keep a Changelog format
• Backfill notable changes from git history for current state (as "Unreleased" or "0.1.0")
• Adopt Semantic Versioning (semver)
• Update scripts/bundle.sh to accept version from environment variable or argument instead of hardcoded value
• Document version bump process in CONTRIBUTING.md
• Consider: conventional commits for automated changelog generation
• Consider: tools like git-cliff or auto-changelog for automation

Technical Notes

• scripts/bundle.sh line sets CFBundleShortVersionString to hardcoded "0.1.0"
• AboutView.swift reads version from CFBundleShortVersionString with "0.1.0" fallback
• generate-build-info.sh already captures git SHA — version should complement this

Acceptance Criteria

• CHANGELOG.md exists with at least current state documented
• Version is no longer hardcoded in bundle.sh
• Versioning strategy is documented