feat(config): add OPENCODE_DISABLE_GLOBAL_CONFIG env var

[kynnyhsap]

Add environment variable flag to disable loading global user configuration, mirroring the existing OPENCODE_DISABLE_PROJECT_CONFIG feature from #8093.

Fixes #10021

Use Case

Running multiple opencode processes with different versions where global config (~/.config/opencode) conflicts with local agent development.

What Gets Disabled

When OPENCODE_DISABLE_GLOBAL_CONFIG=true:

• ~/.config/opencode/ - global config directory (config files, agents, commands, plugins)
• ~/.opencode/ - user home .opencode directory
• ~/.config/opencode/AGENTS.md - global rule file
• ~/.claude/CLAUDE.md - claude compat rule file

Behavior Matrix

Flag Set	Global Config	Project Config	OPENCODE_CONFIG_DIR
Neither	Loaded	Loaded	Loaded (if set)
DISABLE_PROJECT	Loaded	Skipped	Loaded (if set)
DISABLE_GLOBAL	Skipped	Loaded	Loaded (if set)
Both	Skipped	Skipped	Loaded (if set)

Usage

# Disable global config, use only project config
OPENCODE_DISABLE_GLOBAL_CONFIG=true opencode

# Disable both, use only custom config dir
OPENCODE_DISABLE_GLOBAL_CONFIG=true OPENCODE_DISABLE_PROJECT_CONFIG=true OPENCODE_CONFIG_DIR=/path/to/config opencode

Test Plan

• Added 5 new tests for OPENCODE_DISABLE_GLOBAL_CONFIG
• Verified existing OPENCODE_DISABLE_PROJECT_CONFIG tests still pass
• Manual testing with built binary

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