Add explicit self-send option to send tool

[KyleAMathews]

Summary

• add a self: true option to the send tool for sending to the current entity
• document that agents should use self: true with afterMs to schedule future work for themselves
• wire Horton and Worker send tools with their current entity URL

Test

• pnpm --dir packages/agents-runtime test --run test/send-tool.test.ts

[github-actions]

Electric Agents Desktop Builds

Build artifacts for commit fc9ffd7.

Platform	Status	Artifact
macOS Apple Silicon	Passed	DMG
macOS Intel	Passed	DMG
Windows x64	Passed	Installer
Linux x64	Passed	AppImage / deb

Workflow run

[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 55.86%. Comparing base (8803b36) to head (fc9ffd7).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
packages/agents-runtime/src/tools/send.ts	94.28%	2 Missing ⚠️
packages/agents/src/agents/worker.ts	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4398      +/-   ##
==========================================
- Coverage   59.46%   55.86%   -3.61%     
==========================================
  Files         304      259      -45     
  Lines       30626    26920    -3706     
  Branches     8335     7449     -886     
==========================================
- Hits        18211    15038    -3173     
+ Misses      12397    11867     -530     
+ Partials       18       15       -3     

Flag	Coverage Δ
packages/agents	67.52% <0.00%> (ø)
packages/agents-mcp	?
packages/agents-runtime	80.70% <94.28%> (+0.02%)	⬆️
packages/agents-server	74.44% <ø> (ø)
packages/agents-server-ui	6.21% <ø> (ø)
packages/electric-ax	43.81% <ø> (ø)
packages/experimental	?
packages/react-hooks	?
packages/start	?
packages/typescript-client	?
packages/y-electric	?
typescript	55.86% <91.66%> (-3.61%)	⬇️
unit-tests	55.86% <91.66%> (-3.61%)	⬇️

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