feat(config): add OPENCODE_DISABLE_GLOBAL_CONFIG flag

[ZY-Yim]

Issue for this PR

Closes #21264

Type of change

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

What does this PR do?

When embedding OpenCode via opencode serve in a host application (e.g. Electron app), all config is provided through
OPENCODE_CONFIG_CONTENT. But global config files and directories from ~/.config/opencode/ still get loaded and merged, which can
interfere with the intended configuration.

This adds OPENCODE_DISABLE_GLOBAL_CONFIG env var. When set to 1:

• loadGlobal() in config.ts returns {} early, skipping config.json / opencode.json / opencode.jsonc under
  ~/.config/opencode/
• ConfigPaths.directories() in paths.ts excludes Global.Path.config from the returned list, so global agents, skills, commands,
  and plugins are also not scanned

Project-level .opencode/ directories, OPENCODE_CONFIG, and OPENCODE_CONFIG_CONTENT are not affected. Follows the existing
OPENCODE_DISABLE_* naming convention.

How did you verify your code works?

1. Set OPENCODE_DISABLE_GLOBAL_CONFIG=1 + OPENCODE_CONFIG_CONTENT and ran opencode serve — global config loading logs
   (service=config path=~/.config/opencode/...) no longer appear, only loaded custom config from OPENCODE_CONFIG_CONTENT
2. Without the flag — behavior identical to before, no regression
3. Verified .opencode/agents/ and .opencode/skills/ still load correctly with the flag set

Screenshots / recordings

N/A — no UI changes, env var only.

Checklist

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

[github-actions]

Hey! Your PR title feat: add OPENCODE_DISABLE_GLOBAL_CONFIG flag to skip global config loading doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.

[github-actions]

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

Found a potential duplicate:

PR #20784: "feat(sdk): add settingSources to control which config sources are loaded"
#20784

Why it's related: This PR also addresses controlling which configuration sources are loaded. It provides a settingSources mechanism, which may overlap with or be an alternative approach to the OPENCODE_DISABLE_GLOBAL_CONFIG flag being added in PR #21650. Both PRs are trying to solve the problem of selectively loading/excluding config sources, particularly useful for embedded scenarios like opencode serve.