Add CLI commands and template integration for build environment

[JAORMX]

Summary

Add CLI commands and Dockerfile template integration for the build environment configuration added in #2740.

CLI Commands:

• thv config set-build-env KEY value - Set a validated environment variable
• thv config get-build-env [KEY] - Show one or all configured variables
• thv config unset-build-env KEY - Remove a specific variable
• thv config unset-build-env --all - Remove all variables

Template Integration:

• Add BuildEnv field to TemplateData struct
• Update npx.tmpl, uvx.tmpl, go.tmpl to inject ENV directives at top of builder stage
• Modify protocol handler to load build env from config automatically

Example usage:

# Configure custom package mirrors thv config set-build-env NPM_CONFIG_REGISTRY https://npm.corp.example.com thv config set-build-env GOPROXY https://goproxy.corp.example.com # View configuration thv config get-build-env # Run protocol builds (automatically uses configured mirrors) thv run npx://@modelcontextprotocol/server-github

This completes THV-2732 custom package registry support.

Test plan

• Unit tests for template rendering with BuildEnv
• Manual testing of CLI commands (set, get, unset, --all)
• task test passes
• task lint passes

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 57.14286% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 55.82%. Comparing base (b33cf66) to head (19a21fb).
⚠️ Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/runner/protocol.go	57.14%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@ Coverage Diff @@ ## main #2742 +/- ## ========================================== + Coverage 55.81% 55.82% +0.01%  ========================================== Files 315 315 Lines 30598 30605 +7 ========================================== + Hits 17079 17086 +7  Misses 12032 12032 Partials 1487 1487 

☔ View full report in Codecov by Sentry.
📢 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.