fix: hint when sudo cannot find bootit after cargo install

[mincomk]

Problem

Closes #11

After cargo install bootit, running bootit scan as a normal user shows:

Error: This program must be run as root (try: sudo bootit ...)

but following that advice fails:

$ sudo bootit scan
sudo: bootit: command not found

Root cause

cargo install places the binary in ~/.cargo/bin, which is on the user's PATH but not on sudo's secure_path (Debian/Ubuntu default: /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin). sudo resets the environment and only searches secure_path, so it cannot find the binary at all — sudo -E doesn't help either, because secure_path overrides PATH even with -E.

Fix

• src/util.rs — when run as non-root, detect a user-local install (binary under $HOME or in .cargo/bin) and attach an actionable help to the error:

  sudo /home/user/.cargo/bin/bootit ...
  

  or the one-time symlink:

  sudo ln -s /home/user/.cargo/bin/bootit /usr/local/bin/bootit
  

  When the binary lives in a system directory, the error message is unchanged.
• Unit tests for the hint generation (system dir → no hint, cargo install → hint).
• CI — run cargo test in addition to cargo build.
• README — new Troubleshooting section documenting the issue and both workarounds.
• CHANGELOG — [Unreleased] entry.

Verified locally: built clean on Rust 1.97, 3 tests pass, and the rendered error output was checked for both the system-dir and ~/.cargo/bin cases.