[build-tools] Pin the default agent-device version for remote sessions

[gwdp]

Why

agent-device@0.20.4 is broken on npm — it imports @agent-device/ad-script, which isn't published. Hosted sessions installed latest, so the daemon crashed on startup and sessions errored.

How

Default to a pinned 0.20.3 instead of latest. --package-version still overrides. Bump the constant once upstream is fixed.

Test Plan

yarn typecheck, yarn lint, yarn fmt:check, and the startAgentDeviceRemoteSession / startArgentRemoteSession unit tests.

[github-actions]

✅ Thank you for adding the changelog entry!

[codecov]

Codecov Report

❌ Patch coverage is 16.66667% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.43%. Comparing base (b27fe84) to head (3b03769).

Files with missing lines	Patch %	Lines
...c/steps/functions/startAgentDeviceRemoteSession.ts	16.67%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4142      +/-   ##
==========================================
+ Coverage   62.43%   62.43%   +0.01%     
==========================================
  Files        1009     1009              
  Lines       45735    45732       -3     
  Branches     9621     9616       -5     
==========================================
+ Hits        28548    28549       +1     
+ Misses      15726    15722       -4     
  Partials     1461     1461              

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