[build-tools] Execute sandbox commands over MCP connection

[sjchmiela]

Why

Let the Expo MCP server run shell commands in a connected sandbox.

How

• Add exec_command with retained command sessions.
• Add write_stdin for input and incremental output polling.
• Keep sessions across WebSocket reconnects.
• Stop retained processes when the daemon stops.

Test plan

• Added command executor unit tests.
• Added a WebSocket command integration assertion.
• Ran formatting, focused lint, build-tools typecheck, and focused tests.

Stacked on the sandbox daemon message protocol PR.

[github-actions]

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