chore: remove deprecated files and configs

Remove obsolete cspell word lists, editorconfig, vscode settings,
and retired workflows (rebase, stale, sync-labels, wrangler).
Drop legacy installer scripts i.sh and install_alpha.sh.

Author: ss-o

.cspell/project-words.txt

@@ -150,3 +150,7 @@ worker/
 node_modules/
 .pnpm-store/
 .cargo-ok
+AGENTS.md
+CLAUDE.md
+GEMINI.md
+.github/copilot-instructions.md