fix(agentconfig): distinguish IsNotExist from other errors + chore(aep): remove dead code

[aaronwong1989]

Summary

This PR implements two architecture improvements from the selected issues:

1. fix(agentconfig): readFile error handling (fix(agentconfig): readFile swallows all os.ReadFile errors including permission denied #80)

   • Modified readFile to return (string, error) instead of string
   • Properly distinguishes between IsNotExist errors (silent skip) and other errors like permission denied (propagate to caller)
   • Updated loadFile and Load to handle errors via new loadFileWithErrorCount helper
   • Added test case TestLoad_PermissionError to verify error propagation

2. chore(aep): remove dead EncodeChunk function (chore(aep): remove dead EncodeChunk function #75)

   • Removed EncodeChunk function (duplicate of Encode)
   • Removed test cases TestEncodeChunk and TestEncodeChunk_NDJSONSafe
   • The function was only used in tests and provided no value

Verified Already Implemented

The following issues were verified as already implemented in the codebase:

• fix(session): concurrent worker termination (fix(session): sequential worker termination in Close causes O(N×5s) shutdown latency #84) - Already uses errgroup in TerminateAllWorkers()
• fix(worker/ocs): SSE race condition (fix(worker/ocs): readSSE send-on-closed-channel race during termination #83) - Already has sseCancel field and proper mutex protection

Skipped

• chore(config): applyMessagingEnv DRY violation (chore(config): applyMessagingEnv DRY violation — 20 repetitive env var mappings #72) - Skipped due to complexity and risk of introducing bugs

Test Plan

• All existing tests pass (make test)
• Linting passes (make lint)
• Formatting passes (make fmt)
• New test case added for permission error handling
• Quality checks pass (make quality)

Architecture Impact

• agentconfig: Improved error visibility and debugging capability
• aep: Reduced code surface by removing unused function

Related Issues

Closes #80 (partially - error handling only)
Closes #75

Co-Authored-By: Claude Sonnet 4.6 noreply@anthropic.com

[codecov]

Codecov Report

❌ Patch coverage is 67.56757% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.54%. Comparing base (53f7fdd) to head (bfa83c0).
⚠️ Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/agentconfig/loader.go	67.56%	7 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #99      +/-   ##
==========================================
- Coverage   59.58%   59.54%   -0.04%     
==========================================
  Files         134      134              
  Lines       15969    15979      +10     
==========================================
  Hits         9515     9515              
- Misses       5860     5867       +7     
- Partials      594      597       +3     

Flag	Coverage Δ
unittests	59.54% <67.56%> (-0.04%)	⬇️

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