feat: add governance policy linter CLI command

[imran-siddique]

Add a lint-policy subcommand to the agent-compliance CLI that validates YAML policy files for common mistakes: missing required fields, unknown rule types, conflicting rules (e.g., allow and deny same action), deprecated field names. Output: list of warnings/errors with file:line references. Place in packages/agent-compliance/src/agent_compliance/cli/. Use the PolicySchema from schemas/ for validation.

[Jiawen-lee]

Hi I would like to take this issue. Could you assign this to me? Thanks!