fix(init): prevent parent hub contamination during bd init (GH#896)

When running bd init in a subdirectory of a hub (e.g., ~/Repos/project
where ~/Repos/.beads exists), the new database was incorrectly inheriting
issues from the parent hub.

Root cause: checkGitForIssues() computed the relative path from gitRoot
to beadsDir but did not validate that beadsDir was actually inside the
git repository. When beadsDir was outside (e.g., ../.beads), it would
still attempt to import, causing contamination.

Fix: Add a guard to reject beadsDir paths that start with .. (outside
the git repository boundary).

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

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

cmd/bd/autoimport.go

@@ -118,6 +118,13 @@ func checkGitForIssues() (int, string, string) {
 		return 0, "", ""
 	}
 
+	// GH#896: Reject beadsDir that is outside the git repository.
+	// This prevents bd init from inheriting issues from a parent hub
+	// when initializing a new project in a subdirectory.
+	if strings.HasPrefix(relBeads, "..") {
+		return 0, "", ""
+	}
+
 	// Determine which branch to read from (bd-0is fix)
 	// If sync-branch is configured in local config.yaml, use it; otherwise fall back to HEAD
 	// We read sync-branch directly from local config file rather than using cached global config