fix: respect npm config cwd for plugin installs

[j4ng5y]

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 using the wrong cwd for npm config lookup.

I ran into issues trying to install a custom plugin from a private (Artifactory) npm registry. I assumed I wasn't the only one and was correct.

When I was reproducing #21123, the main issue I hit was that plugin/package installs could end up reading npm config from the ambient process.cwd() instead of the OpenCode instance/config directory that actually declared the plugin or provider. That meant private registry settings from .npmrc could get ignored.

While tracing that through, I also found the same problem affecting relative plugin paths like ./some-plugin, which could resolve from the wrong directory for the same reason.

This change makes those install flows use the instance/config directory explicitly instead of relying on whatever the current shell cwd happens to be. It also updates relative/path plugin resolution to use the directory that declared the plugin.

I added tests around the npm config path itself, plugin install flows, provider installs, config installs, and relative plugin resolution.

I think this works because it fixes the actual source of the problem: the install context was wrong. Once the right cwd gets threaded through, npm config resolution and relative plugin resolution both happen in the place they were meant to.

How did you verify your code works?

I ran targeted tests for the affected paths and typecheck (sorry for the mise pollution, but it's how I run things):

mise exec bun@1.3.11 -- bun --cwd packages/opencode test test/npm/npm.test.ts test/plugin/shared.test.ts test/plugin/install.test.ts test/plugin/loader-shared.test.ts test/cli/tui/plugin-install.test.ts --timeout 30000
mise exec bun@1.3.11 -- bun --cwd packages/opencode test test/provider/provider.test.ts test/config/config.test.ts --timeout 30000
mise exec bun@1.3.11 -- bun run --cwd packages/opencode typecheck

Those all passed locally.

Also ... I used it and it fixed my issues 😄

Screenshots / recordings

n/a - not a UI change

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:

Potential Related PRs Found

PR #22025: fix(plugin): load npm config before Arborist installs

• fix(plugin): load npm config before Arborist installs #22025
• Why related: Also addresses npm config loading for plugin installs. Likely a related or overlapping fix for the same npm configuration issue.

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

• fix(opencode): resolve npmrc with @npmcli/config #21697
• Why related: Specifically handles npmrc resolution, which is directly related to the npm config cwd issue being fixed in the current PR.

These PRs appear to be addressing similar issues around npm configuration and plugin installation. Recommend checking their status and scope to ensure there's no overlap.