feat(agent): broker secret scans through sandbox

[radim10]

Summary

• route sandboxed secret-scan hooks through the agent proxy broker
• enforce restricted scan budgets and host-side scan confinement
• add CI coverage and make confinement tests fail instead of skip
• use collision-proof temporary repositories in budget tests

Verification

• cargo test — 631 tests passed
• focused restricted budget tests passed
• Docker sandbox tests passed as part of cargo test