fix: preserve pinned field during JSONL import (bd-phtv)

Fixed two code paths where pinned=false from JSONL would overwrite
existing pinned=true in database:
- importer.go: Only update pinned if explicitly true in JSONL
- multirepo.go: Use COALESCE to preserve existing pinned value

Added tests for pinned field preservation.

Note: Bug may have additional code path - investigation ongoing.

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

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

internal/importer/importer.go

@@ -567,8 +567,11 @@ func upsertIssues(ctx context.Context, sqliteStore *sqlite.SQLiteStorage, issues
 					updates["acceptance_criteria"] = incoming.AcceptanceCriteria
 					updates["notes"] = incoming.Notes
 					updates["closed_at"] = incoming.ClosedAt
-					// Pinned field (bd-7h5)
-					updates["pinned"] = incoming.Pinned
+					// Pinned field (bd-phtv): Only update if explicitly true in JSONL
+					// (omitempty means false values are absent, so false = don't change existing)
+					if incoming.Pinned {
+						updates["pinned"] = incoming.Pinned
+					}
 
 					if incoming.Assignee != "" {
 						updates["assignee"] = incoming.Assignee
@@ -662,8 +665,11 @@ func upsertIssues(ctx context.Context, sqliteStore *sqlite.SQLiteStorage, issues
 				updates["acceptance_criteria"] = incoming.AcceptanceCriteria
 				updates["notes"] = incoming.Notes
 				updates["closed_at"] = incoming.ClosedAt
-				// Pinned field (bd-7h5)
-				updates["pinned"] = incoming.Pinned
+				// Pinned field (bd-phtv): Only update if explicitly true in JSONL
+				// (omitempty means false values are absent, so false = don't change existing)
+				if incoming.Pinned {
+					updates["pinned"] = incoming.Pinned
+				}
 
 				if incoming.Assignee != "" {
 					updates["assignee"] = incoming.Assignee