fix(cli): walk PATH manually for opencode detection (#75)

The previous detection shelled out to which/where, which fails in
environments where those binaries aren't on PATH (Alpine/slim containers,
bunx sandboxes, NixOS, custom wrapper-script setups) — even when opencode
itself is reachable.

Replace with a portable Node-only PATH walker (findOnPath) that handles:
- wrapper scripts (regression for the original reporter)
- symlinks (and ignores broken symlinks)
- empty PATH segments and missing PATH
- Windows .exe/.cmd/.bat/.com extension lookup
- directory entries that happen to share the binary name

Also improve the doctor failure message to surface what was checked, so
users with exotic shim/wrapper setups can share the right context when
detection still fails.

11 new tests cover the regression and edge cases.