Add build environment configuration and validation

[JAORMX]

Summary

Add configuration model and validation for build-time environment variables that will be injected into protocol build Dockerfiles (npx://, uvx://, go://). This enables organizations to configure custom package mirrors for corporate environments.

• Add BuildEnv map to Config struct for storing environment variables
• Add validation functions for env var keys (must be uppercase, no reserved system keys like PATH, HOME)
• Add validation functions for env var values (reject shell metacharacters to prevent injection)
• Extend Provider interface with build env operations (Set, Get, GetAll, Unset, UnsetAll)
• Implement for DefaultProvider, PathProvider, and KubernetesProvider (no-op)
• Add comprehensive tests for validation and provider operations

This is the foundation for THV-2732 custom package registry support. The CLI commands and Dockerfile template integration will follow in a separate PR.

Test plan

• Unit tests for key validation (format, reserved keys)
• Unit tests for value validation (dangerous characters)
• Unit tests for provider operations (PathProvider, KubernetesProvider)
• task test passes
• task lint passes

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 88.63636% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 55.83%. Comparing base (1200343) to head (54884a3).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/config/interface.go	66.66%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2740      +/-   ##
==========================================
+ Coverage   55.73%   55.83%   +0.10%     
==========================================
  Files         314      315       +1     
  Lines       30510    30598      +88     
==========================================
+ Hits        17004    17084      +80     
- Misses      12019    12028       +9     
+ Partials     1487     1486       -1     

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