chore: clippy disallows std::env access outside the config module

[ecalifornica]

Part of #187; with the two PRs below it, closes #187. No user-visible changes.

What

• clippy.toml adds a disallowed-methods rule for std::env::var, var_os, vars, vars_os, set_var, and remove_var. Readers take the value as a parameter; tests construct values instead of mutating the environment.
• The clippy quality gate (scripts/quality_gates.sh, the gate CI runs) adds --all-targets, so the rule covers unit tests, integration tests, examples, and build scripts. Without it, clippy skips test and example code, which is where the env-mutating machinery lived.
• Five sites carry #[expect(clippy::disallowed_methods)] with a reason:
  • path-cli config::search_path: the config module is the one place that reads the environment.
  • path-cli fuzzy::which: the fzf probe keeps its own read until the search-path question on the bottom PR of this stack is decided. A follow-up PR removes this exception.
  • pathbase-client/build.rs: cargo passes OUT_DIR to a build script only through the environment.
  • toolpath-cursor's dump_fixture example and real_session_sanity test: both read the developer's real Cursor store on purpose.
• #[expect] rather than #[allow]: an exception that stops firing becomes a warning itself, so stale exceptions fail the gate.

Verified

• Red-first: an unannotated std::env::var call in src code and in #[cfg(test)] code each fail the gate with the configured reason. The test-code probe passes without --all-targets, so the gate change is load-bearing.
• cargo clippy --workspace --all-targets -- -D warnings is clean on this branch. A stale build cache can mask a new clippy.toml locally; cargo clean -p <crate> refreshes it. Fresh CI checkouts are unaffected.

[github-actions]

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