Migrate built-in Handoff to workflow execution - #197

Summary

• Run the existing Handoff flow through the workflow engine and persist every run as file-backed execution records.
• Keep GUI, CLI, save-progress, context-only, and receiver continuation paths on one implementation.
• Store workflow definitions and run records under ~/.codans/workflows/; remove legacy workflow storage paths and SQLite assumptions.
• Preserve source context, immutable packets, node requests, submissions, events, and real Agent verification.

Validation

• 107 Codans app tests passed in the focused Handoff/workflow selection.
• 7 CodansCore cache/config tests passed.
• Final workflow path test selection: 33 tests passed.
• Real GUI and CLI acceptance completed with Pi Agent, including Handoff, Save Progress, context-only preparation, and receiver continuation.
• git diff --check passed.
• Existing unrelated SwiftLint findings remain in RootFeature and RootFeatureTests.

No SQLite or compatibility migration is included.