fix: apply receiver conflict policy

[vsamarth]

Summary

• thread receiver conflict policy from app config into core receiver planning
• apply Reject and Rename when resolving expected destination paths before accept/export
• preserve resume behavior for already-exported files and cover nested rename cases with regression tests

Root cause

Receiver configuration exposed a conflict policy, but the listener/session/core request path dropped it before normal destination planning. Core then rejected existing destinations directly, so the configured policy never ran.

Impact

Receiver transfers now honor Reject and Rename end to end. Rename remains the effective default across the existing receive surfaces, and conflicts are resolved before transfer begins.

Validation

• cargo fmt
• cargo test

Closes #39.
Closes #27.