Preserve user env settings during provider switch

[istarwyh]

Summary

• Preserve user-managed ANTHROPIC_* and CLAUDE_* env settings unless the active provider defines the same key
• Simplify env cleanup to remove only provider env key conflicts
• Update merge strategy docs and regression tests for the narrower cleanup behavior

Test plan

• go test ./internal/config ./internal/provider
• ./check.sh

🤖 Generated with Claude Code

[guyskk]

Closing: superseded by #76 (already merged on main), which fixes the same issue (user env in settings.json being wiped) with a safer approach.

Empirical finding (claude 2.1.144): settings.json env strictly overrides the process environment variable that ccc passes to the claude subprocess via syscall.Exec. Verified with isolated dual-port tests on ANTHROPIC_BASE_URL.

Consequence: this PR's narrower cleanup (keeping user ANTHROPIC_/CLAUDE_ in settings.json + removing delete(settingsWithHook,"env")) would let stale/user ANTHROPIC_* in settings.json override the provider env passed on the command line, silently breaking provider switching (wrong base_url/token/model). #76's FilterUserEnvForSettings (strip all ANTHROPIC_/CLAUDE_ + managed keys from settings.json) is the correct approach and already preserves non-provider user env.

[guyskk]

fixed in https://github.com/guyskk/claude-code-supervisor/blob/main/CHANGELOG.md#040---2026-05-20