fix(plugin): load npm config before Arborist installs

[ghost91-]

Issue for this PR

Closes #21123

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

This fixes plugin installs from private npm registries.

After the switch to @npmcli/arborist, OpenCode created Arborist without loading npm config first. That caused plugin installs to ignore .npmrc settings such as scoped registries and fall back to registry.npmjs.org.

This PR loads npm config with @npmcli/config and passes the flattened result into Arborist. Npm.add() loads config from Global.Path.cache, which preserves the previous cache-local .npmrc behaviour. Npm.install(dir) loads config from dir, so local config still works there as well.

Related:

• [Bug] .opencode/package-lock.json generated without respecting ~/.npmrc registry configuration #21079
• Npm.outdated() and @npmcli/arborist in src/npm/index.ts ignore user's npm registry config — plugin install hanging in some area #21324
• Plugin install fails to use custom npm registry #21436

How did you verify your code works?

1. Added regression tests for:
   • scoped registry config from user .npmrc
   • cache-root .npmrc
   • install-dir .npmrc
2. Ran:
   • bun test test/npm/index.test.ts
   • bun test test/config/config.test.ts
   • bun test test/plugin/loader-shared.test.ts
   • bun typecheck
3. Built the patched binary locally and verified that installing a plugin from a private scoped registry now works with .npmrc config.

Screenshots / recordings

N/A

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

I found a potentially related PR:

PR #21697: fix(opencode): resolve npmrc with @npmcli/config
#21697

This PR appears related because it also deals with resolving npm config using @npmcli/config. Since PR #22025 (current) is specifically about loading npm config before Arborist installs for the plugin system, while #21697 may address a similar issue in the main opencode codebase, they could be addressing related or overlapping concerns with npm configuration handling.

[cynicalight]

hello, do you know why everybody failed these two test: e2e(linux) and e2e(windows)? 🤣

[cp0213]

Can't wait for this PR to be merged! My opencode has been stuck at version 1.3.13 and I haven't been able to update it for days.