fix(config): clone global cache to prevent cross-workspace mutation leak

[weiconghe]

Issue for this PR

Closes#41916

Type of change

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

What does this PR do?

Config.loadInstanceState merges the process-global config cache into the per-workspace result via remeda mergeDeep. When the workspace has no project-level value for a nested key (e.g. mcp), mergeDeep copies the same object reference from the global cache into the result — verified against remeda 2.26.0:

mergeDeep({}, { mcp: { server1: {...} } }).mcp === global.mcp // true

Plugin config hooks receive the workspace's live config-state object (packages/opencode/src/plugin/index.ts:245) and commonly mutate it in place (config.mcp[name] ??= {...}). When the workspace config shares a nested reference with the global cache, those writes leak back into cachedGlobal — which has Duration.infinity TTL. Every other workspace attached to the same opencode serve process then sees the leaked MCP servers (or any other nested config) in its /mcp status.

Fix:structuredClone the cached global config before merging it into the per-workspace result. The loadGlobal(authEnv) path already returns freshly parsed objects (no caching), so only the getGlobal() path needs cloning.

How did you verify your code works?

Added a regression test in test/config/config.test.ts that:

1. Sets up a global-only config with mcp (no project-level override)
2. Loads the instance config, simulates a plugin config hook mutation (cfg.mcp.injected = {...})
3. Asserts Config.use.getGlobal() does not see the mutation

The test fails without the fix (globalCfg.mcp.injected is the injected object) and passes with it. Full config test suite: 93 pass, 4 pre-existing Windows-specific failures (unrelated — ENOENT on .jsonc auto-creation, Git Bash/Cygwin path timeout, permission key-order on Windows).

Note: the test clears OPENCODE_CONFIG_DIR because a developer's local config may contain mcp: {}, which incidentally breaks the shared reference chain via mergeDeep and masks the bug.

Screenshots / recordings

N/A

Checklist

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