Fix Pinet Escape suppression for encoded key events

[tmustier]

Summary

• Recognize raw Escape plus Kitty CSI-u and xterm modifyOtherKeys plain-Escape encodings in Pinet session UI runtime
• Keep modifier-bearing Escape-like sequences from triggering interrupt drain suppression
• Add coverage for cmux/Ghostty-style encoded Escape events

Tests

• pnpm --filter @gugu910/pi-slack-bridge lint
• pnpm --filter @gugu910/pi-slack-bridge typecheck
• env -u PINET_MESH_SECRET -u PINET_MESH_SECRET_PATH pnpm --filter @gugu910/pi-slack-bridge test
• pre-push hook: pnpm lint && pnpm typecheck && pnpm test