feat(cli): persist durable council results

[ernestprovo23]

Closes #58

Summary

• add opt-in --json-output PATH for long buffered council runs
• atomically persist complete CouncilResult JSON with user-private temp-file semantics
• preserve normal stdout and existing result exit codes; persistence failure renders stdout then exits 1
• reject streaming and invalid destinations before council construction
• document the privacy boundary and implementation design

Verification

• 724 offline tests passed
• 91.1% coverage (required: 75%)
• Ruff check and format clean
• gitleaks clean
• pip-audit: no known vulnerabilities
• independent review: no remaining findings

No provider calls or source-corpus data were used in the test suite.