fix(doctor): improve shell path resolution

[kalvinnchau]

Summary

• fix doctor binary resolution to avoid treating zsh function/alias output as executable paths.
• replace login-shell which with path-only shell lookup: whence -p for zsh and type -P for bash.
• validate resolved candidates as absolute executable files before using them.
• apply the same resolver hardening to shared acp-client and blox-cli command discovery.
• simplify resolver control flow and add focused tests for invalid shell output.

i have git mapped to a function that does some cleanup, but a minimal repro:

git() {
  echo "hi"
  command git "$@"
}

Testing

• cargo fmt -p doctor -p acp-client -p blox-cli
• cargo test -p doctor -p acp-client -p blox-cli