feat(cli): load ~/.deepagents/.env as global dotenv

[Mason Daugherty (mdrxy)]

Closes #1744

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Load ~/.deepagents/.env as a global API key source so users don't have to copy .env files into every project or export secrets in their shell profile. The global file is loaded first with override=False, then the project/CWD .env with the caller's override setting — so project-level values win during /reload while global defaults apply at bootstrap.

Changes

• Add _GLOBAL_DOTENV_PATH (~/.deepagents/.env) and load it in _load_dotenv before the project .env, using override=False so it acts as a fallback layer
• OSError reading the global file (permissions, broken symlink) logs a warning with exc_info and falls through to project .env — matches the existing pattern in _find_dotenv_from_start_path
• Both the module-level bootstrap call and Settings.reload_from_environment go through the same two-tier _load_dotenv, so the behavior is consistent across startup and /reload