Add Config, the single owner of environment reads

[ecalifornica]

Part of #184 (PR 1 of the plan there). path-cli only. No library changes. No user-visible changes.

What

• Config holds every value the CLI reads from the process environment: TOOLPATH_CONFIG_DIR, HOME, USERPROFILE, XDG_DATA_HOME, COPILOT_HOME, APPDATA, PATHBASE_URL, TOOLPATH_QUERY_EXPLAIN.
• Config::load() extracts one immutable value with figment: Serialized::defaults under an env layer restricted to those 8 variables.
• The env layer is VerbatimEnv. It wraps Env and emits values as verbatim strings. Env::data type-infers values and has no option to disable this (the env cargo feature requires parse-value), so TOOLPATH_QUERY_EXPLAIN=1 would arrive as an integer. The Config field types are the single type authority.
• Config::config_dir() resolves the config root from plain values: override, then $HOME/.toolpath, then error.
• Config::home_dir() resolves the home the provider resolvers use: HOME on Linux and macOS, USERPROFILE on Windows. Same order as the resolvers' internal lookup. config_dir() resolves from HOME alone; a USERPROFILE fallback there is a behavior change, out of scope for this series.
• The free config_dir() is transitional. It loads a Config per call and keeps the ~30 call sites unchanged until the command-module PRs pass &Config down from run().
• PATHBASE_URL_ENV moves to config.rs. cmd_pathbase is compiled out of the wasm/emscripten build, and CONFIG_ENV_VARS needs the constant on every target. cmd_pathbase re-exports it.

Tests

• figment::Jail tests pin the env-to-Config mapping. They hold the existing TEST_ENV_LOCK: Jail serializes only against other Jail tests, and other test modules mutate HOME / TOOLPATH_CONFIG_DIR under that lock.
• One Jail test pins verbatim extraction: TOOLPATH_QUERY_EXPLAIN=01 extracts as "01". Type inference turns 01 into the integer 1.
• config_dir precedence and home_dir fallback tests use plain values and do not touch the environment.

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