buzz-acp: scrub env values from agent notification logging

While testing an ACP agent integration, I observed that some agents rebroadcast the user's full MCP server configurations — **including plaintext env values such as API keys** — in custom notifications (observed: grok's `_x.ai/mcp/servers_updated`). Any client or harness that logs raw agent streams (debug tracing, transcript capture, observer feeds) will persist those secrets to disk.

Two-part suggestion:

1. **Defensive (this repo):** redact values for well-known secret-shaped keys (`*_API_KEY`, `*_TOKEN`, `*_SECRET`, …) in any path where buzz-acp or the desktop app logs/relays raw agent notifications.
2. **Root cause (upstream):** agent CLIs shouldn't transmit MCP env values over ACP at all — server name/command suffice for client display. I'll raise this with xAI separately; filing here so the defensive fix isn't coupled to that timeline.

Context: surfaced during #2812 verification (details deliberately omitted here; no secrets were included in that PR's dossier either).

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Start by tracing the buzz-acp and desktop-app paths that log or relay raw agent notifications, including custom notifications such as `_x.ai/mcp/servers_updated`. Identify where notification payloads are persisted or exposed to observers, then inspect any existing coverage there. Done means well-known secret-shaped environment keys have redacted values in every identified logging or relay path, with no plaintext API keys, tokens, or secrets retained.

Written by the indexing model from the issue text.