feat(contract): add multi-sig guardian validation for payout

[DavidAkere204]

• Add guardians list and guardian_threshold fields to Plan struct
• Add GuardianApprovals(Address) variant to DataKey for per-plan storage
• Add approve_payout() function: validates caller is a listed guardian, prevents duplicate approvals, persists approvals, emits guardian/approved event
• Update trigger_payout() to reject with GuardianThresholdNotMet when approvals count < guardian_threshold (skipped when threshold is 0)
• Add NotAGuardian, AlreadyApproved, GuardianThresholdNotMet errors
• Update create_plan() to accept guardians and guardian_threshold params
• Add 4 new tests: 2-of-3 approval flow, duplicate rejection, non-guardian rejection, and zero-guardian passthrough

Closes #840

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@DavidAkere204
Run cargo fmt --all -- --check

and clippy