Fix auto-import git history backfill bug (bd-4pv)

Auto-import was allowing git history backfill to run, which could
incorrectly purge issues. The fix adds NoGitHistory=true to all
auto-import code paths:
- autoimport.go (importFromGit)
- autoflush.go (autoImportIfNewer)
- daemon_sync.go (importToJSONLWithStore)

Git history backfill is designed to detect deletions that happened
after a local DB was created. During auto-import, there is no local
work to protect - we are importing from the authoritative JSONL source.

Also adds comprehensive tests for NoGitHistory behavior.

Generated with Claude Code

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

cmd/bd/autoflush.go

@@ -168,11 +168,11 @@ func autoImportIfNewer() {
 	
 	// Use shared import logic (bd-157)
 	opts := ImportOptions{
-
 		DryRun:               false,
 		SkipUpdate:           false,
 		Strict:               false,
 		SkipPrefixValidation: true, // Auto-import is lenient about prefixes
+		NoGitHistory:         true, // Skip git history backfill during auto-import (bd-4pv)
 	}
 
 	result, err := importIssuesCore(ctx, dbPath, store, allIssues, opts)