chore(gitignore): ignore per-host runtime state and local backups

Runtime state now ignored: agent/blobs (blob store), install-id (per-host
identity), puppeteer/ (browser cache), .AGENTTRASH/, *.save, *.bak-*.
Backups parked in .AGENTTRASH/ per trash convention (never rm user files).

.gitignore

@@ -43,3 +43,13 @@ last-changelog-version
 .DS_Store
 *.swp
 *.tmp
+
+# --- per-host runtime identity + caches (never config) ---------------------
+agent/blobs/
+install-id
+puppeteer/
+
+# --- local backups + staging artifacts: kept on disk, never in history -----
+.AGENTTRASH/
+*.save
+*.bak-*