Phase 1: Edge schema consolidation infrastructure (Decision 004)

Add metadata and thread_id columns to dependencies table to support:
- Edge metadata: JSON blob for type-specific data (similarity scores, etc.)
- Thread queries: O(1) conversation threading via thread_id

Changes:
- New migration 020_edge_consolidation.go
- Updated Dependency struct with Metadata and ThreadID fields
- Added new entity types: authored-by, assigned-to, approved-by
- Relaxed DependencyType validation (any non-empty string ≤50 chars)
- Added IsWellKnown() and AffectsReadyWork() methods
- Updated SQL queries to include new columns
- Updated tests for new behavior

This enables HOP knowledge graph requirements and Reddit-style threading.

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

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

internal/storage/sqlite/schema.go

@@ -43,13 +43,15 @@ CREATE INDEX IF NOT EXISTS idx_issues_assignee ON issues(assignee);
 CREATE INDEX IF NOT EXISTS idx_issues_created_at ON issues(created_at);
 -- Note: idx_issues_external_ref is created in migrations/002_external_ref_column.go
 
--- Dependencies table
+-- Dependencies table (edge schema - Decision 004)
 CREATE TABLE IF NOT EXISTS dependencies (
     issue_id TEXT NOT NULL,
     depends_on_id TEXT NOT NULL,
     type TEXT NOT NULL DEFAULT 'blocks',
     created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
     created_by TEXT NOT NULL,
+    metadata TEXT DEFAULT '{}',    -- JSON blob for type-specific edge data
+    thread_id TEXT DEFAULT '',     -- For efficient conversation threading queries
     PRIMARY KEY (issue_id, depends_on_id),
     FOREIGN KEY (issue_id) REFERENCES issues(id) ON DELETE CASCADE,
     FOREIGN KEY (depends_on_id) REFERENCES issues(id) ON DELETE CASCADE
@@ -59,6 +61,8 @@ CREATE INDEX IF NOT EXISTS idx_dependencies_issue ON dependencies(issue_id);
 CREATE INDEX IF NOT EXISTS idx_dependencies_depends_on ON dependencies(depends_on_id);
 CREATE INDEX IF NOT EXISTS idx_dependencies_depends_on_type ON dependencies(depends_on_id, type);
 CREATE INDEX IF NOT EXISTS idx_dependencies_depends_on_type_issue ON dependencies(depends_on_id, type, issue_id);
+CREATE INDEX IF NOT EXISTS idx_dependencies_thread ON dependencies(thread_id) WHERE thread_id != '';
+CREATE INDEX IF NOT EXISTS idx_dependencies_thread_type ON dependencies(thread_id, type) WHERE thread_id != '';
 
 -- Labels table
 CREATE TABLE IF NOT EXISTS labels (