[EXT] Add JSON schema contracts for extension CLI output

[flyingrobots]

Problem

git mind extension list --json and git mind extension validate --json produce JSON output but there are no schema contracts for them in docs/contracts/cli/.

The contracts test suite (test/contracts.integration.test.js) canaries these commands and validates output against schemas. Extension commands are missing from that coverage.

Work

• Create docs/contracts/cli/extension-list.schema.json
• Create docs/contracts/cli/extension-validate.schema.json
• Add canary tests for both commands to test/contracts.integration.test.js

Ref: #252