fix(schema): add pinned column to schema compatibility checks

The schema_probe.go and doctor/database.go had different column lists
for validating database schemas. Neither included the pinned column
added in migration 023, causing:
- Doctor to report "All required tables and columns present" on
  databases missing the pinned column
- Potential failures when using `gt mail send` on databases without
  the pinned column

Changes:
- Add pinned, sender, ephemeral to doctor's criticalChecks
- Add metadata, thread_id to dependencies check
- Add all missing columns to schema_probe.go's expectedSchema:
  source_repo, close_reason, deleted_at, deleted_by, delete_reason,
  original_type, sender, ephemeral, pinned

Fixes: beads-9yc

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

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

cmd/bd/doctor/database.go

@@ -173,8 +173,8 @@ func CheckSchemaCompatibility(path string) DoctorCheck {
 	// This is a simplified version since we can't import the internal package directly
 	// Check all critical tables and columns
 	criticalChecks := map[string][]string{
-		"issues":         {"id", "title", "content_hash", "external_ref", "compacted_at", "close_reason"},
-		"dependencies":   {"issue_id", "depends_on_id", "type"},
+		"issues":         {"id", "title", "content_hash", "external_ref", "compacted_at", "close_reason", "pinned", "sender", "ephemeral"},
+		"dependencies":   {"issue_id", "depends_on_id", "type", "metadata", "thread_id"},
 		"child_counters": {"parent_id", "last_child"},
 		"export_hashes":  {"issue_id", "content_hash"},
 	}