Fix hook binary resolution for managed installs (#760)

## Summary
- add `--binary` to `roborev init` and `install-hook` so users can explicitly choose the roborev executable baked into git hooks
- resolve stable shim/symlink paths for empirically verified mise and Homebrew installs instead of pinning hooks to versioned binaries
- cover mise Go backend, mise GitHub Releases backend, Homebrew/Linuxbrew, explicit binary overrides, and unsupported-manager non-guesses in tests

## Validation
- `go fmt ./...`
- `go test ./internal/githook`
- `go vet ./...`
- `go test ./...`
- `make lint`
- Docker probes for mise Go backend, mise GitHub backend, Homebrew/Linuxbrew, rtx, and asdf


Co-authored-by: Marius van Niekerk <mariusvniekerk@users.noreply.github.com>