fix(core): keep OPENCODE_CONFIG_DIR additive for global AGENTS.md

[minutechreview]

Issue for this PR

Closes #28658, #32825

Type of change

• Bug fix

What does this PR do?

OPENCODE_CONFIG_DIR is meant to add a second config location, not replace the global one. It does add correctly for agents/commands/skills, because ConfigPaths.directories() always includes Global.Path.config (the static XDG path) regardless of the flag. But the global AGENTS.md lookup in session/instruction.ts and the equivalent lookup in core/instruction-context.ts both build their path from Global.Service's config property, which is Flag.OPENCODE_CONFIG_DIR ?? Global.Path.config — so once you set the env var, ~/.config/opencode/AGENTS.md is never checked again.

I added Global.configDirs(config) in packages/core/src/global.ts, which returns [config, Global.Path.config] (override first so it still wins if it defines its own AGENTS.md, deduped when they're the same value), and pointed both loaders at it instead of duplicating the logic. First-match-wins order is preserved in both call sites.

How did you verify your code works?

• Added unit tests for Global.configDirs and for the extracted globalInstructionFiles helper in session/instruction.ts that assert the override is checked, the real default is still checked when the override doesn't have the file, override still wins when both exist, and no duplicate path when they're the same.
• Added an integration test in core/test/instruction-context.test.ts that writes an AGENTS.md to the real global config dir, points OPENCODE_CONFIG_DIR at an unrelated empty dir, and confirms the global file still loads.
• Confirmed the new tests fail against the pre-fix code (via git stash) and pass against the fix.
• bun test for packages/core (1101 tests) and packages/opencode (3599 tests): all passing.
• bun turbo typecheck for both packages: clean.
• oxlint on all changed files: no new warnings (3 pre-existing unrelated unused-import warnings in instruction.ts are unchanged from dev).

Checklist

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

[hotzen]

this needs to be merged asap please!

[minutechreview]

@hotzen #47468 fixes this. It builds the global config and AGENTS.md paths from the real global config folder instead of the OPENCODE_CONFIG_DIR override, so the global layer (including MCP servers) loads again. It's mergeable against dev, and I'm happy to rebase or adjust anything a maintainer wants.

[hotzen]

so how do we get the maintainer's attention