[CAS-003] CLI JSON schema contracts for content commands

[flyingrobots]

CAS-003: CLI JSON schema contracts

Parent: #271 (M13 VESSEL)
Depends: #273 (CAS-002)

Deliverable

JSON schemas for content CLI output in docs/contracts/cli/:

• content-set.schema.json — output of content set --json
• content-show.schema.json — output of content show --json
• content-meta.schema.json — output of content meta --json

Design

• Follow existing pattern from M9.T1 (e.g., extension-add.schema.json)
• Draft 2020-12 schemas
• Validated in tests via ajv