Extract `opencode_xdg.ts` helper module

[jeonghun-jj-lee]

Problem

Four call sites across the extension hardcode ~/.config/opencode/ and ~/.local/share/opencode/ as literal path.join(os.homedir(), ".config", "opencode", ...) / path.join(os.homedir(), ".local", "share", "opencode", ...) strings. They break under non-standard XDG_CONFIG_HOME / XDG_DATA_HOME and bypass configurable settings.

This slice extracts the shared resolution logic into a single module that all consumers import.

Scope

Create packages/extension/src/opencode_xdg.ts exporting two functions:

• opencodeConfigDir(): string — returns $XDG_CONFIG_HOME/opencode (or ~/.config/opencode when unset)
• opencodeDataDir(): string — returns $XDG_DATA_HOME/opencode (or ~/.local/share/opencode when unset)

Pure utility — no VS Code API dependencies, no side effects.

Acceptance Criteria

• opencodeConfigDir() returns path.join(process.env.XDG_CONFIG_HOME, "opencode") when XDG_CONFIG_HOME is set; otherwise path.join(os.homedir(), ".config", "opencode").
• opencodeDataDir() returns path.join(process.env.XDG_DATA_HOME, "opencode") when XDG_DATA_HOME is set; otherwise path.join(os.homedir(), ".local", "share", "opencode").
• Both functions are exported and importable by other extension modules.
• Unit tests cover: default (env unset), env set to custom path, env set to empty string (treated as unset).

Testing Decisions

Create packages/extension/test/opencode_xdg.test.ts:

• Mock process.env.XDG_CONFIG_HOME / XDG_DATA_HOME and os.homedir().
• Three cases per function: unset → default, set → custom, empty string → default.

Key Decisions

Decision	Rationale
No VS Code API dependency	Keeps the module testable without vscode mock; consumers layer settings on top
Empty-string env treated as unset	XDG spec: unset or empty means default
path.join (not template literals)	Cross-platform safety (though macOS is the primary target)

Constraints & Invariants

• This module MUST NOT read VS Code settings (amicode.configDir, amicode.sessionDatabase) — those are consumer-side overrides, not base resolution.
• No filesystem side effects (no mkdirp, no writes).
• Consumers that need the VS Code setting override apply it themselves on top of these helpers.

Source

Part of #556