buzz-acp: redact MCP env secrets from wire logs and observer feed

[Maxwellimus]

Summary

The ACP harness injects the agent's Nostr secret key as BUZZ_PRIVATE_KEY — an MCP-server env var in the session/new config. That wire message (and any message echoing it back) was serialized verbatim into the acp::wire debug log and published to the in-process observer feed, so the secret appeared in cleartext in both sinks.

This masks MCP-server env values in the logged/observed copies only. The message delivered to the agent over stdin is unchanged — the agent needs the real key to authenticate.

Scope

The observer feed is owner-scoped and the acp::wire debug log is off by default, so this is secret-hygiene / defense-in-depth rather than a broadly reachable leak. Secrets still shouldn't land in logs or an observability stream, and an agent echoing its config back would surface the key wherever those sinks are read.

Change

• redact_wire returns a copy with every env[].value masked (<redacted>), recursively — covering both the outbound params.mcpServers and any inbound echo. Names and all other fields are preserved. Borrows (no clone) when there's no env to mask.
• Applied at every log/observe sink: outbound session/prompt, session/new, notifications, and session/steer; and inbound reads in both read loops.
• Parse-error events publish only the failed line's length, not its content — an unparseable line can't be structurally redacted and could be a malformed echo of the config.
• write_ndjson still delivers the unredacted message to the agent.

Tests

• Config redaction (e2e): drives the real session/new path with a fake agent that echoes the request back. Asserts the agent receives the real key, the secret appears in no observer event, and both the outbound write and the inbound echo are masked.
• Parse-error path (e2e): a malformed secret-bearing line is recorded as a parse error but never surfaces the secret in the feed.
• Unit: redact_wire masks values while preserving shape (built from the real McpServer/EnvVar structs so a serde rename can't silently disable it) and is a no-op for messages without MCP servers.

just ci green.

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