Good first issue: add machine-readable output to `doubt validate`

[alsoleg89]

Problem

doubt validate is useful for humans, while editor and CI integrations need structured findings without parsing terminal copy.

Proposed interface

doubt validate decision.doubt.json --format json

The command should print one JSON object containing:

• valid
• receipt (null for invalid maps)
• metrics
• findings with path, rule, and message

Acceptance criteria

• Valid and invalid maps both produce parseable JSON.
• Exit code remains 0 for valid maps and non-zero for invalid maps.
• Default human-readable output is unchanged.
• Tests cover both outcomes and paths containing spaces.
• README command documentation is updated.
• No runtime dependency is added.

Relevant code: src/cli.js, src/map.js, and test/cli.test.js.