chore(release): AgentCore 0.18.0

[zhanghanduo]

Release

Prepare AgentCore 0.18.0 after PR #79 merged.

• Bump package and lockfile versions to 0.18.0.
• Move changes/79.fix.md into the 0.18.0 changelog entry.
• Record that proxy compatibility was verified for llm-hub deepseek-flash; self-hosted DSV4 and long-task quality remain untested.

This is a MINOR release under docs/versioning.md because compaction and trimming decisions change.

Local verification

• scripts/version.py --check-tag v0.18.0 and scripts/check_version_bump.py --base 958dc5556cddc03a272bb6b61314cea06ce450fd
• Full suite: 2335 passed, 1 skipped
• Ruff and Pyright: clean
• Wheel and sdist built; Twine metadata checks passed
• Wheel installed and imported in a clean temporary environment