fix: unset LHM_*_CONFIG in the hook wrapper

[walter-square]

The Problem

The LHM_GLOBAL_CONFIG / LHM_LOCAL_CONFIG overrides are intended for dry-run ergonomics, but they're also honored during normal hook execution. Anything that injects env vars into the developer's shell before git runs, e.g. direnv loading a .envrc from a freshly cloned repo, could redirect lhm's config sources to attacker-chosen YAML, which then runs via lefthook.

The Fix

This prepends unset LHM_GLOBAL_CONFIG LHM_LOCAL_CONFIG to the generated wrapper-script content before it execs lhm run-hook, so lhm-as-a-hook never sees those vars even when the surrounding shell does. The vars still work for dry-run via the explicit --global-config / --local-config flags.

Testing

• cargo test (146 passing, including wrapper-content assertions).

[alecthomas]

I think instead we should just remove these envars. I haven't used them once since I initially added them.

[alecthomas]

See #40