P2: Define schema/versioning strategy for MCP workflow tool contracts

[Neftedollar]

Parent epic: #236

Problem

Workflow input/output schema evolution risks breaking clients without explicit version policy.

Scope

• Define versioning conventions for tool names and schemas.
• Add migration/deprecation policy and compatibility window.
• Provide tooling/docs for rolling schema upgrades.

Acceptance Criteria

• Clear policy for breaking vs non-breaking schema changes.
• At least one versioned tool example in docs/tests.
• CI guardrails for accidental breaking contract changes.

Non-goals

• Automatic schema migration of client payloads.