Refactor agent CLI tests to eliminate real subprocess calls

[clcollins]

Summary

• Introduces CommandExecutor interface to abstract exec.CommandContext in agentQuery, enabling dependency injection for tests
• Refactors 7 tests (CommandParsing, MultiWordCommand, CommandNotFound, StderrCaptured, PassesEnvVars, PipesStdin, EmptyCommand) to use a mockCommandExecutor — zero subprocess calls, fully self-contained

Test plan

• make test-all passes locally (fmt-check, vet, lint, test, test-race, test-fixtures)
• make plan-check passes
• make readme-check passes
• CI passes on PR

Closes #318

💘 Generated with Crush

[codecov-commenter]

Codecov Report

❌ Patch coverage is 35.71429% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.50%. Comparing base (4e7d42c) to head (7f86ef1).

Files with missing lines	Patch %	Lines
pkg/tui/claude.go	33.33%	8 Missing ⚠️
pkg/tui/model.go	50.00%	1 Missing ⚠️

❌ Your patch status has failed because the patch coverage (35.71%) is below the target coverage (70.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #328      +/-   ##
==========================================
- Coverage   67.60%   67.50%   -0.10%     
==========================================
  Files          49       49              
  Lines        7568     7573       +5     
==========================================
- Hits         5116     5112       -4     
- Misses       2110     2119       +9     
  Partials      342      342              

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.