fix: handle empty env vars in config (prevent relative path bug)

[arena-ai-coding-agent]

Description

In prime-agent's config loading, empty env vars can cause relative path bug.

// Current pattern
if (process.env.XDG_DATA_HOME) { // empty string is falsy in JS, so JS is okay, but check for whitespace
  path = join(process.env.XDG_DATA_HOME, 'prime-agent')
}

// However, if env var is set to whitespace "   ", join("   ", "prime-agent") -> "   /prime-agent" (invalid)

Proposed fix

• Trim env vars and check non-empty after trim
• Validate path is absolute or handle relative case
• Add utility: getEnvPath(var): string | undefined that trims and validates

Location

• Config loading
• Any env var handling for paths

Why

Robustness for Docker/containers where env vars may be set to empty or whitespace.

Can PR.