* feat: add ProxyConfig struct with per-PM skip_commands and legacy fallback
* feat: consolidate proxy config into structured section with backward compat
Replaces flat proxy_mode/proxy_install_only keys with a structured proxy
section supporting per-package-manager skip_commands. Legacy keys are
respected via fallback when user's config lacks the new proxy section.
Removes deprecated experimental_proxy_mode config and flag.
* fix: env var resolution for nested config keys and deduplicate skip command matching
- Add "." to "_" in Viper env key replacer so nested keys like
sandbox.enabled resolve from PMG_SANDBOX_ENABLED (was silently broken)
- Export IsFirstNonFlagArgInList and remove duplicate from proxy_flow.go
- Add table-driven tests for skip command matching with real-world cases
- Remove redundant env var test
* docs: update proxy configuration and env var documentation
Update config.md env var table to reflect new proxy.enabled and
proxy.install_only keys. Add proxy configuration section to proxy.md
covering config structure, per-PM skip commands, CLI flags, and env vars.
* fix: legacy fallback precedence
