fix(desktop): mount buzz-dev-mcp on custom harnesses

[Bowen2024]

Summary

• Custom ACP harnesses (for example DeepSeek dsh) now spawn with the buzz-dev-mcp sidecar, the same path builtins already use for Buzz CLI auth.
• ACP sandboxes that strip *_KEY env drop BUZZ_PRIVATE_KEY from the child shell; session/new injects credentials into the sidecar instead so buzz messages send can publish.
• Presets stay sidecar-less. This does not inject the nsec into the harness definition env or the default agent bash.

Test plan

• Desktop unit tests: custom harness command match, catalog mcp_command, spawn snapshot mounts buzz-dev-mcp; presets remain empty.
• E2E mock save_custom_harness returns mcp_command: "buzz-dev-mcp".
• Live: Buzz Dev + DeepSeek custom harness; agent published to the channel via the MCP shell (buzz messages send no longer dies on missing BUZZ_PRIVATE_KEY in default bash).
• Restart an existing custom-harness agent after this build and confirm it does not fall back to Codex when the harness JSON is present.
• Confirm Cursor / other presets still have no buzz-dev-mcp sidecar.

Made with Cursor

[github-actions]

🔐 Codex Security Review

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