Implement TOML config parser for jr

Create Omni/Jr/Config.hs using the toml package. Parse jr.toml from project root or ~/.config/jr/config.toml. Config structure per _/llm/jr.md: [general] workspace, llm_api_endpoint, llm_api_key_env; [projects.<name>] upstream_path, git_author, git_email. Provide loadConfig :: IO Config function.