fix(shims): resolve Windows batch file syntax error in generated shims (#556)

* fix(shims): resolve Windows batch file syntax error in generated shims

Fixes #555

The Windows .bat shim template contained problematic syntax that broke
CMD batch file parsing:
- goto/label (:found_file) inside parenthesized if (...) block
- Direct %* expansion inside nested for (...) do (...) block

These caused 'was unexpected at this time' errors when running any
Go command through the shims on Windows.

Solution:
- Move file argument detection to a safe subroutine (:detect_file_arg)
- Call subroutine using 'call' instead of inline goto/label
- Add explicit exit code propagation with exit /b %ERRORLEVEL%

This matches the safer approach suggested in the issue report and
ensures shims work correctly in both cmd.exe and PowerShell.

* [AB#555] docs: add GitHub Copilot instructions for AI agents

Provides comprehensive guidance for AI coding assistants including:
- Branch structure and PR guidelines (main vs master)
- Platform-specific constraints (Windows batch file gotchas)
- Development patterns and testing guidelines
- Common tasks and issue resolution checklist

This helps AI agents understand project conventions and avoid
common pitfalls like Windows batch file syntax errors.

* [AB#555] docs: add AGENTS.md for AI coding assistants

Root-level AI agent instructions that complement .github/copilot-instructions.md.
Provides quick-start guidance, architecture overview, and critical Windows
development constraints.

Multiple locations ensure broader AI tool compatibility:
- AGENTS.md (root) - discovered by most AI systems
- .github/copilot-instructions.md - GitHub Copilot specific

* [AB#555] Update gitignore

* docs: add comprehensive testing standards and development guidelines

Enhanced AI agent documentation with:

Testing Standards:
- Testing philosophy (TDD, coverage goals, performance)
- Environment requirements (unset GOENV_DEBUG critical for tests)
- Complete test targets reference (make test, test-quick, test-verbose, etc.)
- Test organization patterns and naming conventions
- Platform-specific testing (Unix/Windows validation)
- Test artifacts locations (.test-results/)
- PR requirements checklist
- Common testing commands quick reference

Directory Structure:
- Complete cmd/ directory reference (12 command categories)
- Complete internal/ directory reference (31 packages)
- Clear separation of concerns and package purposes

This comprehensive documentation eliminates the need to rebuild context
for each PR and ensures consistent development practices across the project.

Author: ChronosMasterOfAllTime