feat: auto-discover .pkgrc / pkg.config.{js,cjs,mjs}

[robertsLando]

Summary

• Auto-discover a dedicated pkg config next to the input file so users no longer need to pass --config every time or pollute package.json. Discovery order: .pkgrc → .pkgrc.json → pkg.config.js → pkg.config.cjs → pkg.config.mjs.
• Pkgrc and package.json now co-exist: package.json supplies name / bin; pkgrc supplies the pkg config. If both define pkg config, pkgrc wins and a warning is logged.
• .mjs is loaded via a genuine dynamic import() (wrapped in new Function to survive the CommonJS downlevel), enabling function-valued options down the road.
• Explicit --config still wins over auto-discovery.

Closes #238.

Test plan

• test-46-pkgrc — .pkgrc with a plain JS input
• test-46-pkgrc-js — pkg.config.js with a plain JS input
• test-46-pkgrc-mjs — pkg.config.mjs (ESM) with a plain JS input
• test-46-pkgrc-overrides-pkg-field — both .pkgrc and package.json#pkg present; pkgrc wins
• Existing test-46-input-package-json* regression checks still pass
• tsc, prettier -c, and eslint lib test all clean

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds automatic discovery and loading of dedicated pkg configuration files located next to the input (entry file or resolved package.json), reducing the need for --config and avoiding package.json pollution.

Changes:

• Implement auto-discovery of .pkgrc / .pkgrc.json / pkg.config.{js,cjs,mjs} with defined precedence, while keeping --config as highest priority.
• Add dynamic loading support for JS/CJS/MJS config files (including ESM via genuine import()), and make discovered config override package.json#pkg with a warning.
• Add test coverage for .pkgrc, pkg.config.js, pkg.config.mjs, and pkgrc-overrides-package.json#pkg, plus documentation updates.

Reviewed changes

Copilot reviewed 16 out of 16 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
lib/index.ts	Adds config auto-discovery, loading logic for JSON/JS/CJS/MJS, and precedence behavior vs package.json#pkg.
lib/help.ts	Updates CLI help text to mention auto-discovered config files.
docs-site/guide/configuration.md	Documents standalone config files, supported filenames, and precedence rules.
test/test-46-pkgrc/main.js	New test verifying .pkgrc auto-discovery next to a JS input.
test/test-46-pkgrc/test-x-index.js	JS input fixture for pkgrc test.
test/test-46-pkgrc/.pkgrc	.pkgrc fixture defining targets.
test/test-46-pkgrc-js/main.js	New test verifying pkg.config.js auto-discovery.
test/test-46-pkgrc-js/test-x-index.js	JS input fixture for pkg.config.js test.
test/test-46-pkgrc-js/pkg.config.js	pkg.config.js fixture defining targets.
test/test-46-pkgrc-mjs/main.js	New test verifying pkg.config.mjs auto-discovery via dynamic import.
test/test-46-pkgrc-mjs/test-x-index.js	JS input fixture for pkg.config.mjs test.
test/test-46-pkgrc-mjs/pkg.config.mjs	ESM config fixture defining targets.
test/test-46-pkgrc-overrides-pkg-field/main.js	New regression test ensuring pkgrc overrides package.json#pkg targets.
test/test-46-pkgrc-overrides-pkg-field/test-x-index.js	JS entry fixture referenced by package.json bin.
test/test-46-pkgrc-overrides-pkg-field/package.json	Fixture with name, bin, and a conflicting pkg.targets (win).
test/test-46-pkgrc-overrides-pkg-field/.pkgrc	Fixture pkgrc defining targets that should override package.json.