v0.21.1 (data-plugin skills overlay + dependency refresh)

Added

• Reference template for layering Anthropic's data knowledge-work plugin on top of contract-governed tools. All three example agents (revenue_agent, growth_agent, ops_agent) carry an opt-in overlay (off by default, enabled via DATA_PLUGIN_PATH). The agent gains the plugin's analyst skills (validate-data, statistical-analysis, explore-data, sql-queries) while every query stays contract-enforced. growth_agent/agent.py is the canonical template; README documents the pattern.
• Security guard: strict_mcp_config=True restricts the session to the governed in-process server only, so the plugin's bundled .mcp.json warehouse servers stay inert and the agent has no ungoverned path around the contract. Skill list deliberately omits data-context-extractor and viz/dashboard skills.

Compatibility

• No library API change — touches only examples/, docs, lockfile, and tooling. Patch bump.
• Opt-in and degrades gracefully — feature-detects plugins/skills/strict_mcp_config on ClaudeAgentOptions, so examples run unchanged on older SDKs and with zero setup.

Internal

• uv lock --upgrade: claude-agent-sdk 0.1.81 → 0.2.87 (overlay API verified intact across the jump), plus langchain 1.3.0→1.3.2, langgraph 1.2.0→1.2.2, duckdb 1.5.2→1.5.3, snowflake-connector-python 4.5.0→4.6.0, mcp 1.27.1→1.27.2, ruff 0.15.13→0.15.15, and others. Full 602-test suite + ruff + ty green.
• prek autoupdate: ruff-pre-commit v0.15.13 → v0.15.15.