Phase 4: Remove deprecated edge fields from Issue struct (Decision 004)

This is the final phase of the Edge Schema Consolidation. It removes
the deprecated edge fields (RepliesTo, RelatesTo, DuplicateOf, SupersededBy)
from the Issue struct and all related code.

Changes:
- Remove edge fields from types.Issue struct
- Remove edge field scanning from queries.go and transaction.go
- Update graph_links_test.go to use dependency API exclusively
- Update relate.go to use AddDependency/RemoveDependency
- Update show.go with helper functions for thread traversal via deps
- Update mail_test.go to verify thread links via dependencies
- Add migration 022 to drop columns from issues table
- Fix cycle detection to allow bidirectional relates-to links
- Fix migration 022 to disable foreign keys before table recreation

All edge relationships now use the dependencies table exclusively.
The old Issue fields are fully removed.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/types/types.go

@@ -41,12 +41,10 @@ type Issue struct {
 	OriginalType string     `json:"original_type,omitempty"` // Issue type before deletion (for tombstones)
 
 	// Messaging fields (bd-kwro): inter-agent communication support
-	Sender       string   `json:"sender,omitempty"`        // Who sent this (for messages)
-	Ephemeral    bool     `json:"ephemeral,omitempty"`     // Can be bulk-deleted when closed
-	RepliesTo    string   `json:"replies_to,omitempty"`    // Issue ID for conversation threading
-	RelatesTo    []string `json:"relates_to,omitempty"`    // Issue IDs for knowledge graph edges
-	DuplicateOf  string   `json:"duplicate_of,omitempty"`  // Canonical issue ID (this is a duplicate)
-	SupersededBy string   `json:"superseded_by,omitempty"` // Replacement issue ID (this is obsolete)
+	Sender    string `json:"sender,omitempty"`    // Who sent this (for messages)
+	Ephemeral bool   `json:"ephemeral,omitempty"` // Can be bulk-deleted when closed
+	// NOTE: RepliesTo, RelatesTo, DuplicateOf, SupersededBy moved to dependencies table
+	// per Decision 004 (Edge Schema Consolidation). Use dependency API instead.
 }
 
 // ComputeContentHash creates a deterministic hash of the issue's content.