Adopt env package from toolhive-core

[JAORMX]

Summary

Third adoption from the shared toolhive-core library per THV-0032 RFC.

The env package provides a testable abstraction for environment variable access:

• Reader interface for dependency injection
• OSReader implementation wrapping os.Getenv
• Generated mocks for testing isolation

API Changes

// Before
import "github.com/stacklok/toolhive/pkg/env"
import "github.com/stacklok/toolhive/pkg/env/mocks"

// After
import "github.com/stacklok/toolhive-core/env"
import "github.com/stacklok/toolhive-core/env/mocks"

No functional changes - the API is identical.

Test plan

• task lint passes
• Affected packages pass tests

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.52%. Comparing base (f40c900) to head (ad84013).
⚠️ Report is 26 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3572      +/-   ##
==========================================
+ Coverage   65.47%   65.52%   +0.05%     
==========================================
  Files         403      403              
  Lines       39633    39779     +146     
==========================================
+ Hits        25951    26067     +116     
- Misses      11678    11700      +22     
- Partials     2004     2012       +8     

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[ChrisJBurns]

Always love seeing a 🔴 diff