Change spawn worker tool instructions to put the bulk of instructions in the initial message not the system prompt

[KyleAMathews]

This makes worker instructions always viewable as system prompts aren't generally visible when looking at agents.

Also cleaner separation — system prompt is "who are you" and initial message is "what you need to do".

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.23%. Comparing base (6aa0186) to head (359fd09).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #4338       +/-   ##
===========================================
+ Coverage   36.09%   68.23%   +32.13%     
===========================================
  Files         172       75       -97     
  Lines       12150     7860     -4290     
  Branches     3977     2377     -1600     
===========================================
+ Hits         4386     5363      +977     
+ Misses       7754     2486     -5268     
- Partials       10       11        +1     

Flag	Coverage Δ
packages/agents	70.78% <ø> (?)
packages/agents-server	73.46% <ø> (ø)
packages/agents-server-ui	?
packages/electric-ax	37.59% <ø> (?)
typescript	68.23% <ø> (+32.13%)	⬆️
unit-tests	68.23% <ø> (+32.13%)	⬆️

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