feat(cli): load comparison defaults from .hclexp.config

[orian]

Summary

• add HCL .hclexp.config files with global, diff, drift, plan, and web sections
• load $HOME/.hclexp.config first and the exact working-directory file second; no parent traversal
• resolve precedence as built-in < home global < home command < workdir global < workdir command < explicit CLI
• preserve explicit boolean overrides in both directions, including -ignore-column-order=false
• apply ignore_column_order to diff, drift, plan, and dump-mode web
• print every loaded config path to stderr only when stderr is a terminal
• fail closed on malformed HCL, unknown settings, and unknown command blocks
• document the file format, lookup rules, precedence, and output behavior

Tests

• unit tests for HOME/workdir layering, global/command precedence, deduplication, invalid and unknown input, CLI precedence, and terminal-only status output
• end-to-end coverage for diff, plan -dump, drift, and web -dump
• end-to-end coverage for explicit true/false overrides, HOME discovery, invalid config, clean JSON, and clean redirected output
• existing subprocess E2Es isolated from developer-local config
• go test ./...
• go test -race ./internal/... ./cmd/...
• go vet ./...
• go build ./...

Closes #242.