Release 0.9.0

What's Changed

• feat(context): enforce prompt assembly budgets (CTX M8) by @digitalelvis in #41
• feat(runtime): parallel tool approval + refund template (M8 P2) by @digitalelvis in #42
• release: v0.9.0 (M8 performance / memory / context) by @digitalelvis in #43

Full Changelog: v0.8.1...v0.9.0