Add versioning, CHANGELOG, and CI/CD workflows

[kudima03]

Summary

• Embeds \$id and version (0.1.0-preview.0.1.0) in PureQL-Specification.json for stable consumer reference URLs
• Adds CHANGELOG.md following Keep a Changelog format with preview versioning convention (major.minor.patch-preview.major.minor.patch)
• Adds CI workflow (validate.yml): validates all samples against the schema on every push and PR
• Adds CD workflow (release.yml): triggered on *.*.* tags — validates samples, checks schema version matches the tag, extracts changelog notes for the release body, then publishes a GitHub Release with PureQL-Specification.json, samples.zip, CHANGELOG.md, and README.md as assets; preview tags are marked as pre-release automatically

Test plan

• Merge this PR then push tag 0.1.0-preview.0.1.0 — verify the release workflow creates a pre-release with all assets attached
• Verify CI runs and passes on this PR
• Verify schema version in the JSON matches the intended tag before tagging