Remove the test env machinery. Add a lint.

[ecalifornica]

Part of #183. Blocked on #184, #185, #186. No in-process code may read env before this starts.

Delete

• ScopedHome, support::env_lock(), config::TEST_ENV_LOCK.
• ScopedPath and cmd_resume's private duplicate, after cmd_resume takes the search path as a parameter.

Convert

• config::search_path() is the single $PATH read. cmd_resume and cmd_export take the value as a parameter.
• fuzzy.rs keeps its $PATH probe behind #[expect]. Where the value should live (a Config field or a threaded parameter) is decided in review; a follow-up PR removes the exception.
• In-process tests construct Config and resolvers directly.
• assert_cmd subprocess tests keep .env(). That call sets the child environment, not the global one.

Guard

• Add a clippy disallowed-methods rule for std::env::{var, var_os, vars, vars_os, set_var, remove_var} across the workspace.
• Add --all-targets to the clippy gate in scripts/quality_gates.sh, so the rule covers tests and examples.
• Allow it with #[expect] in: path-cli's config module, fuzzy.rs (until the search-path decision), pathbase-client's build script (OUT_DIR is cargo's contract), and toolpath-cursor's real-env example and sanity test.
• CI runs -D warnings already, so the rule is active on day one.