chore(release): prepare 0.18.0

[fas89]

Moves the 0.18.0 notes into CHANGELOG.md. This is the only file changed. The notes cover:

• fix(loader): confine $ref to the contract's directory tree #687: $ref is confined to the contract's directory tree.
• feat(api): load a contract exactly as fluid plan sees it #688: a public contract-loading API.
• fix(security): contract SQL runs in DuckDB's sandbox, confined to what the contract may declare #689: contract SQL runs in DuckDB's sandbox.

The breaking changes and their migrations are under Upgrade notes. Every statement was checked against the merged code, and the companion docs are in Agenticstiger/forge_docs#139, which merges after the release.

Merge order: after #688. Once this merges, v0.18.0 is tagged on its merge commit, and release.yml publishes through TestPyPI and then PyPI.

Documentation

• Docs PR: docs: 0.18.0 contract confinement — $ref root, DuckDB sandbox, contract-loading API forge_docs#139 (release notes page RELEASE_NOTES_0.18.0.md plus the feature pages).