fix: harden electric agents cli remote errors

[icehaunter]

Summary

• Harden electric-ax CLI error rendering so string-shaped server errors and empty status text still produce actionable messages.
• Add optional ELECTRIC_AGENTS_PRINCIPAL support that sends the Electric-Principal header for CLI, start-builtin, and the built-in agents entrypoint.
• Document the principal/header env vars and cover the new behavior with focused tests.

Checks

• pnpm -C packages/electric-ax test -- --runInBand
• pnpm -C packages/electric-ax typecheck
• pnpm -C packages/agents test -- entrypoint-lib
• pnpm --filter electric-ax... -r build
• Remote smoke test: missing principal now prints Error: Missing electric-principal; setting ELECTRIC_AGENTS_PRINCIPAL lists entities successfully.

[codecov]

Codecov Report

❌ Patch coverage is 73.77049% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.79%. Comparing base (b6d2a2b) to head (2a7be10).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
packages/electric-ax/src/start.ts	9.09%	10 Missing ⚠️
packages/electric-ax/src/index.ts	85.71%	6 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #4334       +/-   ##
===========================================
+ Coverage   36.11%   68.79%   +32.68%     
===========================================
  Files         172       75       -97     
  Lines       12146     7897     -4249     
  Branches     3975     2387     -1588     
===========================================
+ Hits         4386     5433     +1047     
+ Misses       7750     2453     -5297     
- Partials       10       11        +1     

Flag	Coverage Δ
packages/agents	70.92% <100.00%> (?)
packages/agents-server	73.46% <ø> (ø)
packages/agents-server-ui	?
packages/electric-ax	42.42% <69.81%> (?)
typescript	68.79% <73.77%> (+32.68%)	⬆️
unit-tests	68.79% <73.77%> (+32.68%)	⬆️

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