feat: un-hardcode NPM registry URL and get from npm config

[entr0pia]

Issue for this PR

Closes #11387

Type of change

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

What does this PR do?

This PR removes hardcoded references to https://registry.npmjs.org in several core packages and scripts. Instead, it dynamically retrieves the configured NPM registry using the idiomatic bun shell ($) to execute
pm config get registry.

How did you verify your code works?

Verification steps:

1. Configured a custom registry via
   pm config set registry <custom_url>.
2. Verified that opencode correctly identifies the custom registry in background package management tasks.
3. Confirmed that focused type checks pass in opencode and @opencode-ai/shared.
4. Verified that @opencode-ai/script builds successfully.

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:

Based on my search, I found two potentially related PRs:

1. PR fix: respect npm config cwd for plugin installs #22765: "fix: respect npm config cwd for plugin installs"

   • fix: respect npm config cwd for plugin installs #22765
   • Related because it also deals with respecting npm config settings

2. PR feat(script): respect npm_config_registry for version lookup #11388: "feat(script): respect npm_config_registry for version lookup"

   • feat(script): respect npm_config_registry for version lookup #11388
   • Related because it addresses npm registry configuration, likely addressing a similar use case for custom registries

Both PRs are related to handling npm configuration dynamically rather than hardcoding values, which aligns with the goals of PR #22783. PR #11388 specifically targets npm registry configuration, making it the most closely related.

[github-actions]

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