feat: respect XDG_DATA_HOME and CLAUDE_CONFIG_DIR for path resolution

[troian]

The plugin hardcoded ~/.local/share/opencode/ and ~/.claude/ for all file paths, ignoring XDG_DATA_HOME and CLAUDE_CONFIG_DIR. This prevented running multiple OpenCode instances in parallel with different Claude accounts, since all instances shared the same claude-account-source.txt. Changes:

• credentials.ts: getAccountStateFile() and getAuthJsonPaths() use $XDG_DATA_HOME with fallback to ~/.local/share
• keychain.ts: readCredentialsFile() and writeBackCredentials() use $CLAUDE_CONFIG_DIR with fallback to ~/.claude
• logger.ts: getDefaultLogPath() uses $XDG_DATA_HOME with fallback to ~/.local/share
• Tests added for all new paths; existing tests updated to isolate env vars that now affect path resolution
• README updated with env var docs and parallel instance usage

When unset, behavior is unchanged (same defaults as before).

Summary

Related issue

Testing

Checklist

• PR title follows Conventional Commits (feat:, fix:, docs:, chore:, etc.)
• make all passes locally (runs lint, build, and test)
• Tests added or updated where applicable
• README or docs updated where applicable

[troian]

hey @griffinmartin any chance you can take a look

[griffinmartin]

@troian Sorry I've been pretty busy so haven't been testing all the open PRs as much as I have in the past. I'll get to it this week.

[markthepixel]

I have a similar issue. Running three Claude Team accounts across different project dirs via CLAUDE_CONFIG_DIR + direnv, and the account selection bleeds across all of them because claude-account-source.txt is always written to the hardcoded path.

Ended up pre-writing it from a shell wrapper before each opencode launch as a workaround, but that breaks every update.