[Task]: Implement governance pending mutation lifecycle

[rian-be]

Summary

Implement a first-class pending mutation lifecycle in ModularityKit.Mutator.Governance so governed mutation requests can exist as deferred runtime objects rather than only immediate execution attempts.

Goal

Make pending execution a durable lifecycle in governance, so requests can wait for approval, external checks, scheduling, dependencies, or quota without being reduced to a terminal mutation result.

Scope

What should be implemented?

• Introduce runtime lifecycle handling for MutationRequestStatus.Pending
• Support explicit pending reasons through PendingMutationReason
• Define request transitions for Pending, Approved, Rejected, Canceled, Expired, Superseded, and Executed
• Add APIs or services that can move requests through the pending lifecycle
• Persist lifecycle transitions through MutationRequestDecision
• Define expiration behavior for pending requests
• Define cancellation behavior for pending requests
• Define how pending requests are listed and resolved

Acceptance Criteria

• Pending mutation requests are represented as first-class governance objects
• Pending state is not modeled only as a terminal block or error outcome
• Lifecycle transitions are explicit and recorded in decision history
• Pending requests can be canceled or expired through defined runtime paths
• Governance runtime can list pending requests by state and reason
• The implementation remains inside ModularityKit.Mutator.Governance

Notes

This issue is the runtime realization of ADR-021.

Approval is only one kind of pending flow. The lifecycle must also leave room for:

• external checks
• scheduling
• dependency waits
• quota holds
• manual review

The current runtime covers the generic pending lifecycle and leaves approval-specific workflow behavior for the next issue.

Related ADRs:

• ADR-020: Governance MutationRequest Model
• ADR-021: Governance Pending Mutation Lifecycle
• ADR-022: Governance Request Decisions and Storage
• ADR-024: Governance Runtime Pending Request Handling