KNOWN_BINS only listed Unix/macOS paths, and whichDevin() always shelled
out to /usr/bin/which. On Windows neither resolves, so the provider could
not find `devin.exe` even when it was installed and on PATH. The model
catalog therefore never loaded and the provider silently fell back to the
four hard-coded FALLBACK_MODELS entries.
- Probe the documented Windows install locations (%LOCALAPPDATA%\devin\
cli\bin\devin.exe, the Devin Desktop bundled CLI for both per-user and
machine-wide installs, plus two fallbacks).
- Build the candidate list lazily so a DEVIN_CLI value set after module
load is still honoured.
- Use `where` instead of /usr/bin/which on Windows, and accept multiple
result lines, since `where` prints one path per line.
All existing Unix/macOS paths are preserved.
Tested on Windows 11, Node 24.5.0, pi 0.87.1, Devin CLI 3000.11.3:
`pi --list-models devin` goes from 4 models to 52 with DEVIN_CLI unset.
`tsc --noEmit` and the existing test suite both pass.
