.gitattributes: exclude server/ from git archive

The server/ directory is a separate deployable, not part of the
WordPress plugin. Without this, `git archive` would bundle the
PHP/SQLite server code inside the plugin zip.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitattributes

@@ -1,5 +1,9 @@
 # Files excluded from `git archive` output (gitea's archive endpoint
 # uses git archive, so these don't ship in any release zip).
+#
+# The plugin is what gets installed in wp-content/plugins/. The server/
+# directory is a separate deployable (see server/DEPLOY.md) — it must not
+# ship inside the WordPress plugin zip.
 
 .beads/         export-ignore
 .claude/        export-ignore
@@ -8,3 +12,4 @@
 AGENTS.md       export-ignore
 CLAUDE.md       export-ignore
 steps.md        export-ignore
+server/         export-ignore