chore(release): 0.12.1

[zhanghanduo]

Summary

Prepare the 0.12.1 patch release from the two fixes merged since v0.12.0:

• Anchor Anthropic prompt-cache breakpoints before transient per-call addenda.
• Keep runaway retry guidance accurate when expansion was skipped or reasoning stopped early.

Update the project and lockfile versions, collect both change fragments in CHANGELOG.md, and remove the consumed fragments.

Validation

• python3 scripts/version.py --check-tag v0.12.1
• python3 scripts/check_version_bump.py --base origin/main
• uv sync --frozen --extra dev --offline
• pytest -q: 1609 passed, 1 skipped
• ruff check agent_core tests scripts
• uv run --frozen pyright agent_core
• uv build --offline
• twine check dist/*