[build-tools] Add sandbox MCP connection step

[sjchmiela]

Why

See https://github.com/expo/universe/pull/30766.

How

Added a simple "sandbox daemon" that connects to MCP and an eas/start_sandbox EAS function.

This sets up the basic structure, planning to take care of protocol, handling messages and executing commands will come in future PRs.

Test Plan

Tested with https://github.com/expo/universe/pull/30766 and https://github.com/expo/universe/pull/30763.

<-- GET /sandbox/connect
New sandbox daemon - sandboxId[01a0692f-b464-77c8-8ab0-e2143eec9872] ipAddress[::1]
--> GET /sandbox/connect 101 23ms
[wrangler:info] GET /sandbox/connect 101 Switching Protocols (50ms)

[codecov]

Codecov Report

❌ Patch coverage is 83.65385% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.54%. Comparing base (51cc35e) to head (7fe8c92).

Files with missing lines	Patch %	Lines
...es/build-tools/src/steps/functions/startSandbox.ts	77.36%	12 Missing ⚠️
...kages/build-tools/src/steps/utils/sandboxDaemon.ts	90.00%	5 Missing ⚠️

Additional details and impacted files

@@                        Coverage Diff                         @@
##           stanley/mcp-server-url-context    #4333      +/-   ##
==================================================================
+ Coverage                           65.50%   65.54%   +0.04%     
==================================================================
  Files                                1061     1063       +2     
  Lines                               49338    49442     +104     
  Branches                            10504    10464      -40     
==================================================================
+ Hits                                32313    32400      +87     
- Misses                              15602    16926    +1324     
+ Partials                             1423      116    -1307     

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[github-actions]

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