Add schema-backed NLBoot evidence validation

[Copilot]

• Create JSON Schema for NLBoot evidence at fixtures/schemas/nlboot-evidence.v1.schema.json
• Valid fixture: existing fixtures/sample_nlboot_evidence.json (passes validation)
• Create invalid evidence fixture at fixtures/invalid_nlboot_evidence.json (fails with clear errors)
• Update sourceosctl/commands/nlboot.py: add validate_evidence() + --validate path through inspect_evidence()
• Update sourceosctl/cli.py: add --validate flag to nlboot evidence inspect + new nlboot evidence validate <path> subcommand
• Update tests/test_cli.py: 9 new tests covering validation (valid, invalid, missing, bad JSON, unknown schema, flag, subcommand via main)
• Update README.md: document new commands
• make validate passes (30/30 tests, CodeQL clean)
• No-op commit pushed to trigger CI rerun after workflow approval

[mdheller]

@copilot Scope looks aligned with issue #3, but this PR is still draft and validation is action-required. Please mark it ready when complete and push a no-op commit if needed to rerun checks after workflow approval. Keep scope unchanged and retain make validate evidence.

[Copilot]

@copilot Scope looks aligned with issue #3, but this PR is still draft and validation is action-required. Please mark it ready when complete and push a no-op commit if needed to rerun checks after wor...

Pushed a no-op commit (d6c2200) to rerun checks. make validate passes locally (30/30 tests, CodeQL clean). Scope is unchanged — the PR is ready for review.